From the Department of Molecular Physiology and
Biological Physics and § Department of Pharmacology,
University of Virginia Health Sciences Center,
Charlottesville, Virginia 22906-0011
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ABSTRACT |
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Incorporation of 32P into
telokin, a smooth muscle-specific, 17-18-kDa, acidic (pI 4.2-4.4)
protein, was increased by forskolin (20 µM) in intact
rabbit ileum smooth muscle (ileum) and by 8-bromo-cyclic GMP (100 µM) in -toxin-permeabilized ileum. Native telokin
(5-20 µM), purified from turkey gizzard, and recombinant
rabbit telokin, expressed in Escherichia coli and purified
to >90% purity, induced dose-dependent relaxation,
associated with a significant decrease in regulatory myosin light chain
phosphorylation, without affecting the rate of thiophosphorylation of
regulatory myosin light chain of ileum permeabilized with 0.1% Triton
X-100. Endogenous telokin was lost from ileum during prolonged
permeabilization (>20 min) with 0.1% Triton X-100, and the time
course of loss was correlated with the loss of 8-bromo-cyclic
GMP-induced calcium desensitization. Recombinant and native
gizzard telokins were phosphorylated, in vitro, by the
catalytic subunit of cAMP-dependent protein kinase, cGMP-dependent protein kinase, and p42/44 mitogen-activated
protein kinase; the recombinant protein was also phosphorylated by
calmodulin-dependent protein kinase II. Exogenous
cGMP-dependent protein kinase (0.5 µM)
activated by 8-bromo-cyclic GMP (50 µM) phosphorylated
recombinant telokin (10 µM) when added concurrently to
ileum depleted of its endogenous telokin, and their relaxant effects
were mutually potentiated. Forskolin (20 µM) also
increased phosphorylation of telokin in intact ileum. We conclude that
telokin induces calcium desensitization in smooth muscle by enhancing
myosin light chain phosphatase activity, and cGMP- and/or
cAMP-dependent phosphorylation of telokin up-regulates its
relaxant effect.
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INTRODUCTION |
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The existence of mechanisms that can regulate contraction of smooth muscle independent of changes in cytoplasmic [Ca2+]i is now well established (for a review, see Ref. 1). Thus, although the major mechanism of activating contraction is the rise in [Ca2+]i and consequent phosphorylation of the regulatory myosin light chain (MLC20)1 by the Ca2+-calmodulin-dependent myosin light chain kinase (MLCK) (2, 3), MLC20 phosphorylation and force can also be increased through Ca2+ sensitization, a G-protein-coupled, Ca2+-independent process that inhibits smooth muscle myosin phosphatase (4). Conversely, phosphorylation of site A of MLCK by several kinases in vitro (5, 6) and calmodulin-dependent protein kinase II in vivo (for a review, see Ref. 7) can reduce its affinity for Ca2+-calmodulin, resulting in "Ca2+ desensitization." Another physiologically significant pathway of Ca2+ desensitization is mediated by cyclic nucleotide-activated kinase(s). Although a cGMP-dependent protein kinase (PKG) and smooth muscle relaxants that increase cellular cGMP (such as atrial natriuretic peptide, endothelium-derived relaxing factor (nitric oxide), and exogenous vasodilators) can reduce [Ca2+]i (for a review, see Refs. 8-10), both PKG and the catalytic subunit of cAMP-dependent protein kinase (PKA) can also relax permeabilized smooth muscle at constant [Ca2+]i (11). We have reported that 8-Br-cGMP reverses G-protein-coupled Ca2+ sensitization and accelerates relaxation and dephosphorylation of MLC20 at constant [Ca2+]i (12). The mechanism of the Ca2+-desensitizing effect of cAMP and cGMP is not known, and, in search of it, we identified telokin as the major cytosolic protein phosphorylated under the influence of 8-Br-cGMP.
Telokin was first discovered by Hartshorne and colleagues (13) and found to be identical with the COOH terminus (155-156 amino acids) of smooth muscle MLCK (13), and it is independently expressed in certain smooth muscles (14) through the activities of serum response factor (15) and a smooth muscle-specific promoter located in an intron of the MLCK gene (16). The crystal structure of telokin solved at 2.6-Å resolution showed a characteristic immunoglobulin fold, but the NH2 terminus containing the phosphorylation sites could not be visualized (17). Telokin, also known as kinase-related protein (18), binds to the S1/S2 region of unphosphorylated smooth muscle myosin (19), modulates, in vitro, the oligomerization of MLCK (20), and, also in vitro, inhibits the phosphorylation of myosin by MLCK (19, 20) through competitive inhibition for the MLCK site (21). Telokin also prevents the folding of the 6 S myosin into 10 S conformation (22) and stabilizes filamentous myosin in solution (19); therefore, it has been suggested that stabilization of myosin filaments is a physiological function of telokin in smooth muscle (19).
Our results suggest that the major in situ effect of telokin is desensitization to [Ca2+]i through a mechanism that does not significantly affect thiophosphorylation but accelerates the dephosphorylation of MLC20 and acts synergistically with the Ca2+-desensitizing effect of an 8-Br-cGMP-activated kinase. Preliminary results of some of these findings have been published (23).
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EXPERIMENTAL PROCEDURES |
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Labeling of Intact and Permeabilized Smooth
Muscle--
Longitudinal muscle of the ileum was removed from rabbits
anesthetized by halothane and exsanguinated according to approved animal protocols. Two groups of intact rabbit ileum longitudinal smooth
muscle strips were incubated in Hepes-buffered Krebs solution in the
presence of [32P]PO43 (5 mCi/ml, NEN Life Science Products) at room temperature for 1 h,
followed by the addition of forskolin (20 µM) to one
group. At 10 min, both control and forskolin-treated groups were
rapidly frozen in liquid N2 and then homogenized with a
glass-glass homogenizer in lysis buffer A containing 20 mM
Tris-HCl (pH 7.4), 250 mM sucrose, 5 mM EDTA, 1 mM dithiothreitol, 1 mM
4-(2-aminoethyl)-benzenesulfonyl fluoride (Calbiochem), and 0.1 µM okadaic acid. Aliquots of cell lysates were then mixed
with Laemmli sample buffer (24) for SDS-PAGE (15% acrylamide unless
otherwise stated) analysis and/or subjected to two-dimensional gel
electrophoresis (first dimension pH ranges used were pH 3-10, pH
4-6.5, pH 5-7, and pH 8-10; Pharmalyte, Amersham Pharmacia Biotech)
(25) followed by autoradiography and Western blot analysis. In another
series of experiments, rabbit ileum muscle strips were permeabilized
with Staphylococcal aureus
-toxin based on published
protocols (12, 26, 28) and then incubated in a G10
(Ca2+-free) or pCa 6.3 solution (25, 26) containing 10 mM EGTA and 0.5 mM [
-32P]ATP
(NEN Life Science Products), with or without 100 µM
8-Br-cGMP. Similar procedures as above were carried out to screen for
phosphoproteins showing increased 32P incorporation upon
treatment with 100 µM 8-Br-cGMP.
Identification of Small, Soluble, Acidic Protein(s) in Rabbit Ileum Smooth Muscle-- In order to identify the proteins with increased 32P incorporation upon forskolin or 8-Br-cGMP treatment, we collected the supernatant of ~1 g of rabbit ileum longitudinal smooth muscle (with or without treatment with 20 µM forskolin) homogenized in buffer A and centrifuged at 100,000 × g and partially purified this soluble fraction by anion exchange chromatography (AP-1Q, Waters) using a 200-ml linear gradient of 0-1 M NaCl in 20 mM Tris-HCl (pH 7.4) over 100 min. Fractions were collected at 2 ml/min and 1 min/fraction. Aliquots (50 µl) of each fraction were examined with two-dimensional gel electrophoresis using ampholyte pH 4-6.5, and the fractions containing small (22-24 kDa), acidic (pI 4.2-4.4) proteins detected by silver staining were pooled and concentrated 5-fold using a centrifuge filter (CentriPrep 3, Amicon). A 15-µl aliquot was then analyzed by two-dimensional gel electrophoresis together with an in vivo labeled tissue lysate to determine whether these small acidic proteins co-migrated with forskolin- and 8-Br-cGMP-stimulated phosphoproteins. The small acidic proteins in the two-dimensional gels were subjected to in-gel trypsin digestion, and peptides eluted from the gel matrix were sequenced by LC mass spectrometry (Biomolecular Research Facilities, University of Virginia).
Purification of Native and Recombinant Telokins--
Native
telokin was purified from frozen turkey gizzards by chromatography on
DEAE-anion exchange followed by phenyl-Superose (Amersham Pharmacia
Biotech) according to the published method (13). Recombinant rabbit
telokin was expressed from a pET vector expression plasmid (a generous
gift of Dr. Paul Herring, Indiana University) in Escherichia
coli strain BL21 (DE3) pLysS (Novagen). Expression of telokin was
induced by 1 mM isopropyl -D-thiogalactoside at a cell density of A650 = 0.3. After 2 h,
cells were harvested and broken open by a French press pressure cell at
18,500 p.s.i. The soluble fraction was purified by isoelectric
precipitation on ice at pH 5.0 followed by centrifugation at
15,000 × g, and the supernatant was adjusted to pH 7.4 before DEAE-anion exchange chromatography, which resulted in
recombinant telokin that was over 90% pure. Both native and
recombinant telokin were identified and characterized by characteristic
blue staining of acidic proteins with Stains-all (13), Western blotting
with a rabbit polyclonal antibody raised against purified turkey
gizzard telokin (a generous gift of Drs. David Hartshorne and Masaaki
Ito), and two-dimensional gel electrophoresis (pH 4-6.5).
Concentrations of purified telokin were determined with the BCA protein
assay kit (Pierce). We note that the Bradford (Bio-Rad) assay uses
Coomassie Blue, which binds much less to acidic proteins than to
standard bovine serum albumin.
In Vitro Kinase Assays--
Phosphorylation of native and
recombinant telokin was determined in kinase assay systems using the
following reagents: catalytic subunit of cAMP-dependent
protein kinase (5-10 units/reaction, New England Biolabs; 1 unit is
the amount of enzyme required to transfer 1 nmol of phosphate to
Leu-Arg-Arg-Ala-Ser-Leu-Gly (Kemptide) in 1 min at 30 °C),
cGMP-dependent protein kinase (5000-10,000 units/reaction,
Calbiochem; 1 unit is the amount of enzyme required to transfer 1 pmol
of phosphate to a peptide substrate, GTRGRRNSI, in 1 min at 30 °C),
p42/44 mitogen-activated protein (MAP) kinase (27), or
calmodulin-dependent protein kinase II (New England Biolabs). Assays with the commercially available kinases were carried
out exactly according to protocols provided by the manufacturers, with
telokin (0.2 mg/ml) as the substrate at room temperature. Reaction with
activated p42/44 MAP kinase (27) was carried out in a buffer containing
50 mM Hepes (pH 7.4), 1 mM -mecaptoethanol, 0.039 mg/ml activated kinase, 0.2 mg/ml telokin, 10 mM
MgCl2, 0.25 mM [32P]ATP (10 mCi/mmol) at room temperature. Reactions were stopped at desired time
points by mixing an aliquot (10 µl) of the reaction with 4× Laemmli
buffer (24) (10 µl) and boiling for 5 min. Proteins were separated by
SDS-PAGE and visualized by autoradiography.
Permeabilization and Storage of Smooth Muscle Strips--
Rabbit
ileum longitudinal smooth muscle strips (3-4 mm long, 400-600 µm
wide) were permeabilized with S. aureus -toxin using published protocols (12, 26, 28) or by 0.1% Triton X-100 in an
intracellular solution containing 1 mM EGTA and no added Ca2+ (G1) for indicated periods of time. For storage,
muscle strips were permeabilized for 20 min with 0.1% Triton X-100 in
G1, immersed in a relaxing solution with 50% glycerol, and stored at
20 °C. Stored strips were washed three times with G1 solution just
prior to use at room temperature. Strips could be stored up to 4 weeks. Details of solutions have been published previously (25, 26, 29).
Western Blot Analyses--
Treated or control smooth muscle
tissues were frozen with liquid N2 and homogenized with
buffer A. Approximately 20 µg of total protein, determined by the
Bradford assay (Bio-Rad), from each tissue homogenate was subjected to
15% SDS-PAGE. Proteins were electrophoretically transferred to PVDF
(Millipore Corp.) membranes. Post-transfer gels were stained with
Coomassie Blue. PVDF membranes were incubated with 0.2% glutaraldehyde
in phosphate-buffered saline containing 0.05% Tween 20 (PBST) for 30 min and then blocked with 5% nonfat dry milk in PBST for 1 h.
After PBST washes (10 min), the PVDF membranes were incubated with
anti-telokin antibody at a 1:5000 dilution in PBST (~1 µg/ml) for
3 h at room temperature or overnight at 4 °C. Following PBST
washes (20 min), incubation with a secondary anti-rabbit IgG conjugated
with horseradish peroxidase (Amersham Pharmacia Biotech) at 1:8000
dilution in PBST for 1 h at room temperature, and final PBST
washes (15 min), the immunoreactive proteins were visualized on film
with enhanced chemiluminescent detection (ECL, Amersham Pharmacia
Biotech). ECL detections were carried out according to the
manufacturer's directions, and the normal exposure time of the x-ray
film (DuPont) to PVDF blots was 30 s. Ready gels (4-20%;
Bio-Rad) were used for immunoblotting with anti-PKG and anti-PKA
antibodies. Two anti-PKG antibodies were used: a goat polyclonal
anti-bovine PKG as a generous gift from Dr. Thomas Lincoln (University
of Alabama at Birmingham) and a rabbit polyclonal anti-human PKG type
I and I
purchased from Upstate Biotechnology, Inc. A rabbit
polyclonal anti-PKA (NH2-terminal residues 7-21 of the
human
-isoform of the catalytic subunit of PKA) was also purchased
from Upstate Biotechnology, Inc. Immunoblotting procedures were
according to the protocol provided by Dr. Padmini Komalavilas, which
uses nitrocellulose instead of PVDF, for the goat anti-bovine PKG and
the manufacturer's directions for the rabbit polyclonal anti-PKG and
anti-PKA antibodies. Secondary antibodies were donkey anti-goat IgG
(Jackson Laboratories) and goat anti-rabbit IgG (Amersham Pharmacia
Biotech) conjugated with horseradish peroxidase.
Measurements of MLC20 Phosphorylation and
Thiophosphorylation--
The smooth muscle strips used for recording
tension were frozen at selected time points by quickly transferring the
specimen to Freon 22 precooled with liquid N2 and cutting
the frozen strips away from the hooks on the tension recording
apparatus. To identify phosphorylated and nonphosphorylated
MLC20, tissue homogenates were separated by two-dimensional
gel electrophoresis with ampholyte pH 4.5-5.4, transferred to
nitrocellulose membranes, and stained with AuroDye (Amersham Pharmacia
Biotech) as described previously (25). Protein spots were analyzed by
densitometry (Molecular Analyst, Bio-Rad). The percentage of
MLC20 phosphorylation or thiophosphorylation was expressed
as (P1 + P2)/(U + P1 + P2) (where U represents unphosphorylated,
P1 represents singly phosphorylated, and P2
represents doubly phosphorylated). To measure telokin-induced dephosphorylation of MLC20, muscle strips were contracted
with pCa 6.2 or pCa 6.0 solutions, with or without telokin treatment at
10 min, and frozen at 15 min after exposure to Ca2+. For
thiophosphorylation studies, stored rabbit ileum smooth muscle strips
were incubated in Ca2+-free, ATP-free solution at room
temperature for 9 min, followed by ATP-free pCa 6.2 solution with 0.5 µM calmodulin in the presence or absence of telokin (20 µM). Thiophosphorylation was initiated by the addition of
1 mM ATPS (Boehringer Mannheim), followed by incubation
for 1 or 5 min as indicated. Strips were then washed with
Ca2+-free, ATP-free solution, and subsequently force was
measured upon exposure to G1 solution containing 4.5 mM
ATP.
Statistics-- Student's t test was performed as indicated under "Results" (Microsoft Excel). All values are means ± S.D. unless otherwise indicated, and n is the number of observations.
Materials-- All reagents were purchased from Sigma unless otherwise specified.
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RESULTS |
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Protein Phosphorylation in Smooth Muscle Induced by 8-Br-cGMP or
Forskolin--
Both 8-Br-cGMP (100 µM) and forskolin (20 µM) induced protein phosphorylation in, respectively,
-toxin-permeabilized and intact rabbit ileum smooth muscle (Fig.
1) and markedly increased 32P
incorporation into a group of small (22-24 kDa upon SDS-PAGE), soluble, acidic (pI 4.2-4.4) protein(s) (Fig.
2, A and B). There was also a significant, but lesser, amount 32P
incorporation into these same proteins in control intact muscle, indicating a basal level of phosphorylation. Analysis of the partially purified rabbit ileum smooth muscle (with or without 20 µM forskolin treatment) cytosolic proteins "spiked"
with the 32P-labeled tissue lysates (see "Experimental
Procedures") showed that the two more acidic spots on two-dimensional
gels detected by silver staining (vertically smeared) exactly matched
32P-containing spots shown by autoradiography (data not
shown). An aliquot (60 µl) of the concentrated partially purified
intact rabbit ileum smooth muscle cytosol, enriched with the small,
acidic proteins, was then subjected to another two-dimensional gel
electrophoresis, and the second dimensional gel was stained with
Coomassie Blue. At least six faintly stained, distinct, small spots
were detected, four of which were subjected to in-gel trypsin
digestion. Six eluted peptides sequenced by LC mass spectrometry
unambiguously matched the sequence of telokin (Table
I). Western blotting of the
two-dimensional gel of the 32P-labeled muscle lysate, using
a rabbit polyclonal anti-telokin antibody, further confirmed that each
of the group of small, acidic, phosphoproteins consisted of telokin
(Fig. 2C).
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Isolation of Turkey Gizzard Telokin-- Telokin was isolated from frozen turkey gizzards by the method published (13), and purified telokin migrated as a group of at least three protein bands of 24-26 kDa on a 15% SDS-PAGE gel (Fig. 3A); each band was an acidic protein as shown on the two-dimensional gel (pI ~4.4), stained blue with Stains-all, and was recognized by the anti-telokin antibody (data not shown); the heterogeneity of avian telokin was recently characterized (30) and probably accounts for the appearance of multiple bands upon SDS-PAGE.
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Expression and Purification of Recombinant Telokin-- We also expressed a rabbit clone of telokin using a pET vector in BL21 (DE3) pLysS E. coli cells and purified the protein based on the method used for isolation of turkey gizzard telokin with some modifications (see "Experimental Procedures"). A yield of 10 mg of telokin/liter of culture was achieved, and the purified recombinant telokin was also consistently detected as multiple bands (22-24 kDa) on one-dimensional 15% SDS-PAGE gels (Fig. 3A) and as multiple spots (pI ~4.4) on two-dimensional gels by silver staining and/or Western blotting. The difference between the apparent molecular weights of native (avian) and recombinant (rabbit) telokins has been reported (14) and arises from small differences in amino acid sequences including three different translational initiation sites as well as COOH-terminal glutamate residues in the avian protein (30). Following storage at 4 °C, native gizzard telokin migrated as one band upon SDS-PAGE (Fig. 3B) without an apparent decrease in molecular mass (~26 kDa), whereas recombinant telokin migrated as one major band around 17 kDa (Fig. 3B). Sequencing the 17-kDa recombinant telokin by Edman degradation (Protein Sequencer; Applied Biosystems, Inc.) showed that the NH2-terminal 10 residues were FLEAVAEEKP, indicating that the 34 NH2-terminal amino acids were missing. The 17-kDa recombinant telokin (amino acids 35-155) was also an acidic protein (pI 4.2-4.4) and stained blue by Stains-all (data not shown), suggesting that it retained the COOH-terminal glutamate residues.
In Vitro Phosphorylation of Telokin by PKA, PKG, MAP Kinase, and Calmodulin-dependent Protein Kinase II-- Both turkey gizzard and recombinant telokin were good substrates for the catalytic subunit of PKA in vitro, but the time course of recombinant telokin phosphorylation was much faster (reaching a stoichiometry of ~0.7 mol of phosphate/mol of protein in 8 min) than that of native telokin (reaching a stoichiometry of ~0.7 mol of phosphate/mol of protein in 32 min) (Fig. 4). This difference in phosphorylation may be due to a basal level of phosphorylation in native gizzard telokin. The holoenzyme of cGMP-dependent protein kinase (activated by 0.5 µM 8-Br-cGMP) phosphorylated turkey gizzard and recombinant telokin slowly to a stoichiometry of 0.4 mol of phosphate/mol of protein in 64 min (Fig. 4). Mass spectrometry of rabbit ileum smooth muscle telokin peptides eluted from two-dimensional gels showed a possibly phosphorylated serine residue (with an additional mass of ~80Da) on one of the six peptides sequenced (Table I, underlined S on peptide 4), which was a proline-directed consensus site and suggested phosphorylation of telokin by MAP kinase in vivo. Indeed, an activated p42/44 MAP kinase phosphorylated both turkey gizzard telokin and recombinant telokin in vitro (Fig. 4). Within the region of the primary structure of rabbit telokin that differs from avian telokin, there is a threonine residue in the context of RLET (absent from avian telokin) that may be a consensus phosphorylation site for calmodulin-dependent protein kinase II. A catalytic subunit of calmodulin-dependent protein kinase II phosphorylated, much more effectively, the purified recombinant than turkey gizzard telokin, in vitro (Fig. 4). None of the kinases could phosphorylate the 17-kDa recombinant telokin (residues 35-155), produced by storage at 4 °C (data not shown), indicating that all of the phosphorylation sites discussed above are within the NH2-terminal 34 amino acids.
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Telokin-induced Calcium Desensitization Associated with MLC20 Dephosphorylation in Triton X-100-permeabilized or Stored Smooth Muscle-- Both isolated turkey gizzard telokin and recombinant telokin (5-15 µM) induced dose-dependent relaxation of submaximal tension induced by Ca2+ in stored rabbit ileum smooth muscle strips (see "Experimental Procedures") (Fig. 5). Following permeabilization with 0.1% Triton X-100 for 30 min or after storage, the recombinant telokin (20 µM) also induced significant relaxation (30 ± 7.5% (n = 3) of the respective pCa 6.2 tension response) in femoral artery, a smooth muscle that, like aortic smooth muscle (14, 16), contains only trace amounts of endogenous telokin (Fig. 6). Of the tissues examined by Western blot analysis using the rabbit polyclonal antibody raised against purified turkey gizzard telokin, rabbit bladder, intact ileum, vas deferens, portal vein, and chicken amnion contained similar high levels of telokin (Fig. 6); rabbit mesenteric artery, femoral artery, cerebral artery, skeletal muscle, and brain tissue showed no detectable level of telokin (Fig. 6); and a marked decrease in telokin content was found in Triton X-100-permeabilized ileum (Fig. 6). Telokin (20 µM) did not relax the submaximal tension generated by ~40% thiophosphorylated MLC20 in stored rabbit ileum smooth muscle strips (data not shown), suggesting that telokin-induced relaxation is associated with dephosphorylation of MLC20.
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Lack of Effect of Telokin on the Rate of Thiophosphorylation of the
Regulatory Myosin Light Chain--
To further verify that the
reduction of MLC20 phosphorylation by telokin was due to
activation of SMPP-1M rather than inhibition of MLCK, we used ATPS
as substrate, because MLC20 thiophosphorylation resists
dephosphorylation (31). The rate of MLC20
thiophosphorylation was not affected by telokin: at 1 min, 35 ± 7.0% in control and 36 ± 6.5% in telokin-treated tissue
MLC20; at 5 min, 47 ± 0.1% in control and 49 ± 4.5% in telokin-treated tissue MLC20 (n = 3 each, p > 0.05; values are means ± S.E.).
These results suggest that telokin has no major effect on MLCK
activity, in vivo, in thiophosphorylating
MLC20.
Loss of Endogenous Telokin and 8-Br-GMP-dependent
Relaxation by Heavy Permeabilization--
Although 8-Br-cGMP induced
significant calcium desensitization of force and MLC20
dephosphorylation in -toxin-permeabilized rabbit ileum and femoral
artery smooth muscles treated with 10 µM A23187 to
eliminate Ca2+ transport by intracelluar organelles (12),
the calcium-desensitizing effect in both tissues was greatly diminished
when they were permeabilized with 0.1% Triton X-100 in G1 solution for
10 min or longer. In
-toxin-permeabilized rabbit ileum smooth muscle
strips where the amount of endogenous telokin was 97 ± 9.8%
(n = 10) (Fig. 7) of that
in intact strips (p > 0.05), reflecting the small 2-nm pores allowing passage only of molecules of 1000 Da or less (28), 50 µM 8-Br-cGMP induced 77 ± 2.2% relaxation of the
tension response to pCa 6.0 (Fig. 7). Following permeabilization with
0.1% Triton X-100 for 10 min, 50 µM 8-Br-cGMP induced
37 ± 15.5% relaxation (Fig. 7). In addition, a significant loss
of endogenous telokin was observed by Western blot analysis during the
same time period (Fig. 7). Following more prolonged (20-min)
permeabilization with 0.1% Triton X-100, 50 µM 8-Br-cGMP
induced 32 ± 9.3% relaxation (Fig. 7), while 17 ± 5.2%
telokin remained in the muscles (Fig. 7). After 40 min of 0.1% Triton
X-100 permeabilization (data not shown) or overnight storage in 50%
relaxing/glycerol solution at
20 °C, there was a further loss of
endogenous telokin (Fig. 7), and 8-Br-cGMP-dependent
relaxation was also significantly decreased further to 15 ± 3.2%
of the respective tension response (Fig. 7). Furthermore, the rate of
8-Br-cGMP-induced relaxation became significantly slower (Fig. 7) along
with the loss of endogenous telokin. The increase in 32P
incorporation into small, soluble proteins stimulated by 100 µM 8-Br-cGMP in
-toxin-permeabilized smooth muscle
(Figs. 1 and 2A) was also abolished in stored muscles or in
muscles permeabilized with 0.1% Triton X-100 for 40 min (Fig.
8A, inset).
Purified telokin (up to 20 µM) did not affect
Ca2+-induced tension in
-toxin-permeabilized smooth
muscle, indicating that its effect required intracellular penetration
(n > 3, data not shown).
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Mutual Enhancement of Relaxation by Activated PKG and
Telokin--
To determine whether the significant reduction of the
relaxant effect of 50 µM 8-Br-cGMP on stored rabbit ileum
smooth muscle was due to inhibition or loss of endogenous PKG during
permeabilization and storage, we added 0.5 µM PKG
activated by 50 µM 8-Br-cGMP to stored muscle strips.
However, this activated exogenous PKG did not affect the pCa
6.0-induced tension (Fig. 8A, right part), suggesting that loss of PKG substrate(s) rather than PKG was
responsible for the markedly diminished relaxant effect of 8-Br-cGMP in
the stored fibers. Furthermore, Western blotting analyses using both the goat polyclonal antibody raised against bovine lung PKG and the
rabbit polyclonal antibody raised against human PKG type I and I
indicated no significant loss of endogenous PKG during permeabilization
and storage of rabbit ileum smooth muscle and significantly less
abundance of PKG in intact ileum, bladder, and vas deferens (not shown)
than that in femoral artery, mesenteric artery, and protal vein (Fig.
8B); PKA content was very similar in the different smooth
muscles tested (Fig. 8B), but a significant loss of PKA was
observed in stored rabbit ileum smooth muscle. Recombinant telokin (10 µM) relaxed pCa 6.0-induced tension of stored muscle
strips by 31 ± 6.1%, and the subsequent addition of PKG (0.5 µM) activated by 8-Br-cGMP (50 µM) induced
a further 41 ± 7.7% relaxation. However, without previous
addition of telokin, PKG (0.5 µM) activated by 8-Br-cGMP
(50 µM) relaxed the stored muscle strips by only 22 ± 7.3%, whereas the subsequent addition of telokin (10 µM) induced further relaxation of 52 ± 5.0% (Fig. 8, A and C). Therefore, telokin and activated PKG
enhance mutually their relaxant effects. Activated PKG (0.5 µM) phosphorylated exogenous telokin added to the stored
muscle strips (Fig. 8A, inset). The shortened
17-kDa telokin (amino acids 35-155) (Fig. 3B) could not be
phosphorylated by PKA/PKG, and when we added to stored muscle strips
this short telokin (10 µM) in the presence of activated
PKG (0.5 µM) by 8-Br-cGMP (50 µM), no
significant mutual enhancement of relaxant effects was observed (Fig.
8C).
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DISCUSSION |
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The major new findings of this study are that telokin accelerates relaxation and dephosphorylation of the MLC20 in smooth muscle, that the loss of endogenous telokin parallels the loss of the relaxant effect of 8-Br-cGMP and PKG, and that the relaxant effects of 8-Br-cGMP and telokin are synergistic. Furthermore, phosphorylation of telokin, measured by 32P incorporation, was very significantly enhanced by forskolin in intact smooth muscle and by 8-Br-cGMP in permeabilized smooth muscle. In conjunction with earlier studies (12), these findings lead to the hypothesis that phosphorylation of endogenous telokin contributes to Ca2+ desensitization by PKG and/or PKA, which is mediated by activation of smooth muscle myosin phosphatase.
In vitro phosphorylation of telokin by PKA has been previously reported (13), and after our studies were completed, we were informed of a preliminary report also indicating such phosphorylation in situ (32). The serine 15 phosphorylated by an 8-Br-cGMP-dependent kinase corresponds to site B of the cyclic nucleotide kinase (PKG and PKA) phosphorylation site of the COOH terminus of MLCK and is, therefore, responsible for the functional effects observed in our studies. An additional serine-phosphorylated peptide within a proline-directed consensus site detected in situ was consistent with MAP kinase phosphorylation, and both MAP kinase and calmodulin-dependent protein kinase II phosphorylated telokin in vitro; the functional effects, if any, of phosphorylation of these latter sites, remain to be determined. None of the above kinases phosphorylated the NH2-terminally truncated telokin (see "Results"), indicating that the amino acids phosphorylated under our experimental conditions were consistent with the consensus phosphorylation sites identified within the first 35 NH2-terminal amino acids of the MLCK/telokin sequence (33).
Several cytosolic and membrane-associated proteins are phosphorylated by cyclic nucleotide-activated kinase(s), and some of these may be involved in regulating cytosolic Ca2+ (for a review, see Ref. 9). There is also considerable evidence showing that agents that increase cyclic nucleotide levels can reduce cytosolic [Ca2+] (for a review, see Refs. 8 and 9). Therefore, it is likely that both mechanisms, reduction in [Ca2+]i and Ca2+ desensitization, make variable contributions to relaxation of smooth muscle induced by agonists that increase cyclic nucleotide levels. However, cyclic nucleotides and their regulated kinases can also relax smooth muscle independently of [Ca2+]i (11, 12, 34-36) (Ca2+ desensitization), and we had suggested that increased activity of smooth muscle myosin phosphatase is at least one of the mechanisms of desensitization (1, 12, 37). Our attention was drawn to telokin as a possible mediator of Ca2+ desensitization, because we found it to be the most abundant protein showing increased 32P incorporation when intact or permeabilized smooth muscles were stimulated with, respectively, forskolin or 8-Br-cGMP.
Desensitization of smooth muscle contraction to Ca2+ can be mediated, however, by inhibition of MLCK (2) and/or activation of SMPP-1M (12, 37). Telokin inhibits MLC20 phosphorylation by MLCK in vitro (19, 38) through competition with MLCK for a common binding site in the S1/S2 region of myosin (21). Should telokin also act in situ primarily through this mechanism, we would expect it to reduce the rate of thiophosphorylation of MLC20. Because we did not detect such an effect, we examined in greater detail the hypothesis that Ca2+ desensitization induced by telokin is mediated by activation of myosin dephosphorylation.
Telokin (5-20 µM) (both the wild type, isolated from turkey gizzard, and the recombinant rabbit protein) relaxed submaximally contracted, permeabilized smooth muscle and accelerated dephosphorylation of MLC20 at fixed [Ca2+]. The concentration of endogenous telokin in phasic smooth muscles is 70-90 µM (19), and the relaxant effect of telokin became greater in parallel with the loss of endogenous telokin, suggesting that, at least in permeabilized smooth muscle, endogenous telokin may have a Ca2+-desensitizing activity even in the absence of exogenous cyclic nucleotide.
That the effect of telokin is enhanced by its phosphorylation by a cGMP- or cAMP-dependent protein kinase is supported by three observations: 1) the significant increase in telokin phosphorylation during relaxation induced by forskolin or 8-Br-cGMP; 2) the concomitant reduction of the effect of 8-Br-cGMP and PKG with the loss of endogenous telokin from permeabilized smooth muscle; and 3) the greater than merely additive relaxant effects of combined PKG and telokin (Fig. 8). The exact mechanism of these interactions is not clear. A "short telokin" that lacks the phosphorylation sites had a significant relaxant effect, but 8-Br-cGMP and PKG did not potentiate its activity, and this suggests that phosphorylation enhances, but is not required for, the Ca2+-desensitizing action of telokin. Similarly, neither phosphorylation with PKA nor removal of the phosphorylatable amino terminus affects the binding of telokin to myosin, and that binding depends largely on the acidic COOH terminus (21). The rapid loss of telokin from permeabilized smooth muscle (Fig. 7) indicates that, unlike calmodulin, an acidic protein of similar molecular weight (39, 40), telokin is not strongly bound to nondiffusible proteins. Depletion of endogenous telokin had no detectable effect on contractility,2 suggesting that telokin, although facilitating myosin assembly in vitro (19), is not required for the maintenance of myosin filaments in situ.
Telokin is a high abundance protein in phasic, but not in tonic, smooth muscles (Refs. 14 and 16; this study). Therefore, it remains to be determined whether Ca2+ desensitization is a function of telokin specific to phasic smooth muscle or whether another acidic protein, perhaps a yet to be detected isoform, plays a similar role in tonic smooth muscles. The higher phosphatase activity of phasic than tonic smooth muscle (41) may reflect its higher telokin content, and the decrease in endogenous phosphatase activity observed following storage of chicken gizzard muscle fibers (42, 43) may also have been in fact due to loss of telokin.
A single, identical cyclic nucleotide kinase-directed consensus site on MLCK is phosphorylated in the presence of calmodulin by both PKA and PKG (13, 44). This is the only such consensus site on telokin, and both PKA and PKG phosphorylated telokin (Fig. 4). Forskolin, an agent that increases cAMP, but not cGMP, also increased telokin phosphorylation in intact intestinal smooth muscle (this study), either through activation of PKA or cross-activation of PKG (for a review, see Ref. 45). Nitroprusside (up to 1 mM) had no relaxant effect on rabbit ileum longitudinal smooth muscle,3 consistent with the reported insensitivity of cyclic GMP production to nitric oxide donors in intestinal, as contrasted to vascular, smooth muscle (46). These findings suggest that both cGMP- and cAMP-activated kinase(s) can mediate telokin phosphorylation and associated Ca2+ desensitization. Given the cross-activation of the two cyclic nucleotide-activated kinases by, respectively, cAMP and cGMP, the question as to which specific kinase is acting under physiological conditions remains to be answered, and the answer may vary among different types of smooth muscles and activators of adenylate and guanylate cyclases. The cGMP-dependent protein kinase can mediate cAMP-induced reduction of [Ca2+]i (47). Recent evidence obtained with relatively specific kinase inhibitors suggests that PKG mediates cAMP-induced phosphorylation of inositol 1,4,5-trisphosphate receptor (48) and Ca2+ desensitization in rat vascular smooth muscle (49).
In view of the activity of unphosphorylated, recombinant telokin, we conclude that phosphorylation is not required for, but only enhances, the Ca2+-desensitizing effect of telokin, perhaps by dissociating it from a bound, less active complex. Given the stoichiometry of telokin (70-90 µM) versus MLCK (2-4 µM (19, 41), we also consider it unlikely that the less active state of nonphosphorylated telokin is due to its binding to MLCK (20, 38). Our results support the hypothesis (1, 12) that PKG and/or PKA and telokin cooperate in a Ca2+-desensitizing mechanism mediated by enhancement of MLC20 dephosphorylation. It remains to be determined whether telokin interacts directly with the regulatory subunit of SMPP-1M (for a review, see Ref. 1) or binds to MLC20 in a manner that facilitates the activity of the catalytic subunit of SMPP-1M.
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ACKNOWLEDGEMENTS |
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We thank Drs. David J. Hartshorne (Arizona) and Masaaki Ito (Mie University, Japan) for providing the antibody to telokin, Dr. Paul Herring (Indiana University) for providing the telokin clone, and Drs. Thomas Lincoln and Padmini Komalavilas (University of Alabama) for providing the goat anti-PKG antibody. Dr. Jackie Corbin (Vanderbilt) generously provided cGMP-dependent protein kinase. We are grateful to Barbara Nordin and Jama Coartney for expert help with the preparation of the manuscript and figures.
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FOOTNOTES |
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* This work was supported by National Institutes of Health Grants PO1-HL48807 and PO1-HL19242.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ To whom correspondence should be addressed.
1
The abbreviations used are: MLC20,
regulatory myosin light chain; 8-Br-cGMP, 8-bromo-cyclic GMP; ATPS,
adenosine 5'-O-(3-thiotriphosphate); PVDF, polyvinylidene
fluoride; PKA, cAMP-dependent protein kinase; PKG,
cGMP-dependent protein kinase; MAP, mitogen-activated
protein; MLCK, myosin light chain kinase; PAGE, polyacrylamide gel
electrophoresis.
2 X. Wu, T. A. J. Haystead, R. K. Nakamoto, A. V. Somlyo, and A. P.Somlyo, unpublished observations.
3 X. Wu, A. V. Somlyo, and A. P. Somlyo, unpublished observations.
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REFERENCES |
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