(Received for publication, August 6, 1996, and in revised form, September 22, 1996)
From the Department of Medical Biochemistry and
Biophysics, Umeå University, S-901 87 Umeå, Sweden and § Center
for Thrombosis, Hemostasis, Atherosclerosis and Inflammation Research,
Academic Medical Centre, University of Amsterdam,
1105 AZ Amsterdam, The Netherlands
Previous studies had pointed to an important function of a putative exposed loop in the C-terminal domain of lipoprotein lipase for activity against emulsified lipid substrates. This loop contains 3 tryptophan residues (Trp390, Trp393, and Trp394). We have expressed and characterized lipase mutants with tryptophan to alanine substitutions at positions 55, 114, 382, 390, 393, and 394 and a double mutant at residues 393 and 394. The substitutions in the N-terminal domain (W55A and W114A) led to poor expression of completely inactive lipase variants. Heparin-Sepharose chromatography showed that mutant W114A eluted at the same salt concentration as inactive wild-type monomers, indicating that this substitution prevented subunit interaction or led to an unstable dimer. In contrast, all mutants in the C-terminal domain were expressed as mixtures of monomers and dimers similarly to the wild-type. The dimers displayed at least some catalytic activity and had the same apparent heparin affinity as the active wild-type dimers. The mutants W390A, W393A, W394A, and W393A/W394A had decreased reactivity with the monoclonal antibody 5D2, indicating that the 5D2 epitope is longer than was reported earlier, or that conformational changes affecting the epitope had occurred.
The mutants W390A, W393A, W394A, and W393A/W394A had decreased catalytic activity against a synthetic lipid emulsion of long-chain triacylglycerols (IntralipidR) and in particular against rat lymph chylomicrons. The most pronounced decrease of activity was found for the double mutant W393A/W394A which retained only 6% of the activity of the wild-type lipase, while 70% of the activity against water-soluble tributyrylglycerol was retained. In the case of chylomicrons also the affinity for the substrate particles was lowered, as indicated by severalfold higher apparent Km values. This effect was less prominent with the synthetic lipid emulsion.
We conclude that the tryptophan cluster Trp390-Trp393-Trp394 contributes to binding of lipoprotein lipase to lipid/water interfaces. Utilizing different lipid substrates in different physical states, we have demonstrated that the tryptophan residues in the C-terminal domain may have a role also in the productive orientation of the enzyme at the lipid/water interface.
Lipoprotein lipase (LPL)1 belongs to
the mammalian triacylglycerol lipase gene family, which also contains
pancreatic lipase and hepatic lipase (1). Based on the crystal
structure of pancreatic lipase and sequence homologies, it can be
assumed that LPL has a similar three-dimensional structure with an
N-terminal folding domain containing the active site and a C-terminal
folding domain consisting of -structures (2, 3, 4). Active LPL is a noncovalent dimer, which readily dissociates into inactive monomers (3, 4, 5, 6). A site for interaction with the low density lipoprotein
receptor-related protein (LRP) has been localized to the C-terminal
domain of LPL (7, 8). This region is also involved in the interaction
with lipoproteins (9). The LRP-binding site and the lipoprotein-binding
site were found to partly interfere with each other (10). Important
structures for both sites appear to be present in, or close to, a
predicted exposed loop in the C-terminal domain round amino acid
residue 388. This region is sensitive to chymotrypsin (9). Cleavages of
the peptide bonds between Phe388-Ser389 and
Trp390-Ser391 in bovine LPL (two residues
longer than human LPL) resulted in reduced activity with synthetic
emulsions of long-chain triacylglycerols, and activity against
chylomicrons was totally abolished (9). It was shown that the
truncated LPL did not bind to chylomicrons. In contrast, the cleavage
had little effect on the activity toward short-chain substrates like
tributyrylglycerol (9).
In the present study we have investigated the role of tryptophan residues in the C-terminal domain of LPL for binding to lipid substrates. We chose to replace the tryptophans by alanine, since it is known that these residues are well suited to mediate interactions to a lipid/water interface (11) and since there is a cluster of presumably exposed tryptophan residues in the C-terminal domain of LPL.
Lipoprotein lipase was purified from bovine milk (12). Human apolipoprotein CII was isolated from plasma by adsorption to IntralipidR, followed by gel filtration and ion exchange chromatography as described (12). IntralipidR (10%) was from KABI Pharmacia Parenterals, Stockholm, Sweden. Heparin-Sepharose was made as described previously (13). [9,10-3H]Trioleoylglycerol was synthesized by Dr. Lennart Krabish, Lund University, Lund, Sweden. The labeled trioleoylglycerol was incorporated into IntralipidR 10% by custom emulsion preparation at Pharmacia Hospital Care, Stockholm, Sweden. Tributyryl glycerol and bovine serum albumin (fraction V) were from Sigma. Labeled tributyryl glycerol (glyceryl-tri-[1-14C]butyrate, 8.0 mCi/mmol) was custom synthesized by American Radiolabeled Chemicals, Inc., St. Louis, MO. Rat lymph chylomicrons, labeled in vivo in triacylglycerols by feeding 14C-labeled oleic acid to cannulated rats, were kindly provided by Dr Roger Savonen, Department of Medical Biochemistry and Biophysics, Umeå University. The chylomicrons were recovered after ultracentrifugation as described previously (14). The concentrations of triacylglycerols in the preparations were measured using the Triglycerides/GB Kit (no. 450032) from Boehringer Mannheim. The 5D2 monoclonal antibody against bovine LPL was a generous gift from Dr. John Brunzell, Department of Medicine, University of Washington, Seattle, WA. Chicken antibodies against bovine LPL were made as described previously (15). Peroxidase-labeled rabbit anti mouse IgG (A2554) was from Sigma. Altered Sites mutagenesis kit was from Promega. Dulbecco's modified Eagle's medium was obtained from Life Technologies, Inc. PcDNAI expression vector was from Invitrogen.
Site-directed Mutagenesis and Transient Expression in COS CellsA 2.4-kilobase PstI-XbaI fragment
from a lipoprotein lipase clone (a kind gift of Dr. R. Lawn, Genentech
Corp.) extending from nucleotides 1 to 2413, was cloned into the
pSelect phagemid vector (Promega). Site-directed mutagenesis was
performed with the Altered Sites site-directed mutagenesis kit from
Promega. C-terminal deletion mutants were constructed by replacement of tryptophan codons with alanine codons using the following primers: M55A, 5-ATCCATGGCGCCACGGTAACA-3
; M114A, 5
-TTTATCAACGCCATGGAGGAG-3
; M382, 5
-AAGCTCAAAGCCAAGAGTGAT-3
; M390A, 5
-TACTTTAGCGCCTCAGACTGG-3
; M393, 5
-TGGTCAGACGCCTGGAGCAGT-3
; M394A, 5
-TCAGACTGGGCCAGCAGTCCC-3
; M393/4A, 5
-TGGTCAGACGCCGCCAGCAGTCCC-3
.
Following mutagenesis, the LPL cDNA was cloned into an eukaryotic
expression vector (pcDNAI, Invitrogen). COS cells were cultured in
12-cm plates until 60-70% confluence was reached. Transfection was
performed using the DEAE-dextran method as described previously (16).
At 24 h post-transfection, heparin (0.25 unit/ml) was added. Media
were collected 24 h later and stored at 70 °C.
Media were collected 48 h after
transfection and were immediately frozen at 70 °C. Available
amounts of each expression medium (50-240 ml) were passed over small
columns of heparin-Sepharose (2 ml) at 4 °C. To avoid the risk of
contamination, a new gel was used for each LPL variant. The flow rate
was 1 ml/min, and binding of LPL activity was almost complete. The
columns were washed with 35 ml of 20 mM Tris-Cl, pH 7.4 containing 20% (w/v) glycerol and 1 mg of bovine serum albumin/ml.
Elution was by linear gradients from 0 to 1.6 M NaCl in
the same buffer (50 ml + 50 ml). Fractions of 5 ml were collected. They
were immediately analyzed for activity against the sonicated
Intralipid-based substrate to localize the peaks of active dimers, and
were then frozen at
20 °C. Recovery of LPL activity over the
columns was between 60 and 70%, and recovery of LPL protein mass was
usually around 75%. The recoveries were higher when higher amounts of
starting material was available (volume of expression medium and
expression level).
Four different assays were used. One was described previously and is based on Intralipid into which 3H-labeled trioleoylglycerol is incorporated by sonication (17). This assay was used for analyses of column fractions. In another assay the only difference was that the emulsion was custom prepared at Pharmacia Hospital Care, where a small batch of IntralipidR 10% was made also containing 3H-labeled trioleoylglycerol. The advantage of this emulsion is that it is stable for years if properly stored, and it is therefore better suited for comparative kinetic studies than the sonicated emulsion. A third assay used rat chylomicrons as substrate. Both the second and the third assay contained (in a total volume of 200 µl): triacylglycerols from Intralipid or from chylomicrons corresponding to 1 mg/ml unless otherwise stated, 0.15 M Tris-Cl, 50 mg of bovine serum albumin/ml, 15 IU heparin/ml, 0.15 M NaCl at pH 7.4. The fourth assay with tributyryl glycerol contained (in a total volume of 200 µl): 0.15 M Tris-Cl, 0.15 M NaCl, 15 IU heparin/ml, 2.0 mg of bovine serum albumin/ml at pH 8.5. For the assays against long-chain triacylglycerols, the fatty acids were extracted based on the method of Dole as described (17). For the tributyrin assay, the released fatty acids were extracted according to the method of Belfrage and Vaughan (18).
All activity measurements were made within 2 weeks after purification
of the LPL dimers. During this time no appreciable loss of catalytic
activity occurred as compared with the initial assay before storage of
the fractions at 20 °C. Comparative analyses of activities against
the three different substrates were made on the same day and on
material that had only been thawed once.
LPL protein in culture media and in column fractions was measured by two different enzyme-linked immunoassays (ELISAs). In both affinity-purified chicken IgG (chicken 225) was used for capture of the antigen. Monoclonal 5D2 antibodies were used together with peroxidase-labeled rabbit anti-mouse IgG for one ELISA as described previously (19), while affinity-purified and peroxidase-labeled IgG from chicken 224 were used for detection in the second ELISA (20). For each measurement three dilutions of each sample were analyzed, and usually a mean value from these three measurements was used. Purified human LPL from post-heparin plasma was used as standard in both assays.
CalculationsFor curve fitting by nonlinear regression and calculations of values for Vmax and Km, the program FigP (BIOSOFT, Cambridge, United Kingdom) was used.
Fig. 1
shows a model of LPL based on the structure of pancreatic lipase (3).
Inspection of the model, in which tryptophan residues are highlighted,
reveals that there are two regions which are enriched in tryptophan
residues. One is located in the N-terminal domain, around the catalytic
site, while the another is located in the C-terminal domain, close to
and in an exposed surface loop. For the present study the following
tryptophan residues in the C-terminal domain were replaced by alanine
residues: Trp382, Trp390, Trp393,
and Trp394. A double mutant, W393A/W394A, was also
constructed. In addition, two of the residues in the N-terminal domain,
Trp55 and Trp114, were replaced by alanine
residues. From the model it appeared as if all the targeted tryptophan
residues were exposed on the surface of the molecule. Both wild-type
LPL and the mutants were transiently expressed in COS cells. The
expression levels varied from 26 ng/ml culture medium to 596 ng/ml
(Table I). The lowest expression levels were obtained
with mutant W114A, and the highest were obtained with wild-type
LPL.
|
To allow comparison of specific catalytic activities of
the different mutants, we purified and concentrated LPL dimers from the
culture media by chromatography on heparin-Sepharose (Fig. 2). Fractions from the columns were analyzed for LPL
activity and LPL mass, to localize the peaks of active dimers eluting
later in the gradients well separated from inactive LPL monomers, which eluted in an earlier peak. Some of the substitutions affected the
reactivity with the 5D2 monoclonal antibody, and we therefore made
parallel assays using a polyclonal antibody for detection as well, as
was described previously (20). For most mutants the monomer/dimer ratio
was slightly altered as compared with wild-type LPL. Interestingly,
mutant W382A appeared to be more stable than the others, since it gave
mostly dimers (Fig. 2B). Pilot experiments showed that all
mutants except W55A and W114A had catalytic activity. Separation of the
expression medium for W114A revealed that it contained only inactive
LPL monomers (not shown). Therefore mutants W55A and W114A were not
further studied.
Reactivity with the Monoclonal Antibody 5D2
It is clear from Fig. 2 (C and D) that W390A and for W393A/W394A reacted poorly with the monoclonal 5D2, while the immunoreactivity of wild-type LPL and W382A was similar in the two ELISAs (Fig. 2 and Table II). This is interesting since the substituted residues are outside the proposed epitope (residues 396-405) for 5D2 (21). Additionally, mutants W393A and W394A showed decreased reactivity to 5D2 as compared with the polyclonal chicken IgG (Table II). Therefore, in the experiments described below, only data from the polyclonal ELISA were used for calculations of specific catalytic activities.
|
Since we knew that some of the mutations were likely to interfere with the lipid-binding ability of LPL, and thus with the apparent substrate affinity, comparisons of the catalytic efficiencies were made as a function of substrate concentration in assays with IntralipidR and with the rat chylomicrons. Replacements of residues Trp390, Trp393, and Trp394 by alanine dramatically decreased the catalytic efficiency of LPL. The double mutant W393A/W394A was the least active both with the synthetic lipid emulsion (Intralipid) and with rat lymph chylomicrons. In contrast, with both substrates the specific activity of mutant W382A was higher than that of wild-type LPL. Mutant W382A was about 30% more active against both IntralipidR and chylomicrons than wild-type LPL. The mutants W390A, W393A, and W394A had lower activity with IntralipidR as substrate than with chylomicrons. This was true also for the double mutant W393A/W394A, which had only 6% of the activity against the synthetic emulsion as compared with wild-type LPL.
Catalytic Activity against Short-chain TriacylglycerolsIt was previously known that the activity of LPL against short-chain triacylglycerols, like tributyrylglycerol, was not affected by truncation of the lipase in the C-terminal domain (9). It was concluded that the truncation affected the lipid-binding ability of LPL or its ability to orient itself at a lipid/water interface. We therefore investigated here whether replacement of tryptophans for alanine residues in the C-terminal domain of LPL had any effect on the activity against tributyrylglycerol (Table III). This substrate is much more soluble than the long-chain triacylglycerols and can therefore be used at non-aggregating substrate concentrations (here we used 0.09 mM). At higher concentrations (>0.4 mM at 37 °C) tributyrylglycerol forms aggregates, but presumably with other interfacial properties than those of long-chain triacylglycerols and phospholipids.
|
At 0.09 mM tributyryl glycerol, all mutants were more than 70% active compared with wild-type LPL and the activity for the double mutant W393A/W394A was more than 85% of that of the wild-type (Table III). Thus, the low catalytic activities with the long-chain triacylglycerol substrates were not due to general loss of catalytic function.
At 2.5 mM tributyrylglycerol, the pattern changed somewhat
and the double mutant W393A/W394A had the lowest specific activity, which was 58% of the specific activity of wild-type LPL (Table III).
Interestingly, also with tributyrylglycerol, the mutant W382A was more
active than wild-type LPL (about 15%). We cannot rule out the
possibility that this was due to an underestimation in the mass
measurement of W382A. It is notable that in this system the specific
activities of wild-type LPL, even with the higher substrate
concentration, are only a few percent of that with emulsions of
long-chain triacylglycerols. This is not the case when
tributyrylglycerol is emulsified with gum arabic and the released fatty
acids are determined by titration (9). That type of assay is, however, not sensitive enough for the amounts of lipase used here. A summary of
the relative activities of all the mutants against the different substrates is shown in Fig. 3.
Comparison of the Apparent Affinities for Lipid/Water Interfaces
The apparent Km values of lipases for substrate emulsions are thought to reflect the affinity of the lipase for the lipid/water interface, rather than its affinity for individual substrate molecules at the interface (22). The substrate saturation curves (data not shown) were used for calculation of apparent Km values for chylomicrons and for IntralipidR, respectively (Table III). With IntralipidR there were only small differences for the different LPL variants. Thus, the large effect on the specific catalytic activity of, e.g., W393A/W394A could not be explained by a change in the apparent affinity for the emulsion droplets. With chylomicrons the Km values for wild-type LPL and for the mutant W382A were similar and comparable with those obtained with IntralipidR. With the other mutants there was a 5-8-fold increase in the Km values as compared with wild-type LPL, indicating that the abilities of these mutants to bind to the chylomicron surface was reduced.
Tryptophan residues are well suited to mediate interactions of proteins with lipid/water interfaces due to their amphipathic properties (11). Their structure with a large hydrophobic double ring and a nitrogen atom able form hydrogen bonds allows both hydrophobic and hydrophilic interactions. It is known that tryptophan residues play an important role in the stabilization and conformation of transmembrane helices in several integral membrane proteins (11).
Molecular modeling of LPL on the structure of pancreatic lipase reveals that there are two regions of exposed tryptophan residues (3). One is located in the N-terminal domain, around the catalytic site. Because of their location, these residues might contribute to the interaction of LPL with lipid/water interfaces (23). The second region is located in the C-terminal domain close to and in an exposed loop. This loop is sensitive to chymotrypsin (24). Cleavage of the peptide bonds between Phe388-Ser389 and Trp390-Ser391 generates a truncated form of LPL, in which catalytic activity against relatively soluble substrates is retained while activity against chylomicrons is abolished (9). According to the model of van Tilbeurgh et al. (3), this exposed loop is located in the same plane as the part of the N-terminal domain that is likely to bind to lipid. There is evidence that the two domains cooperate for proper lipid binding and orientation of LPL at lipid/water interfaces (4, 23). Disturbances of the structure of the lid covering the entrance to the active site, either by cleavage with trypsin (24) or by site-directed mutagenesis (25), have similar effects on the catalytic activity of LPL as disturbances of the structure in the C-terminal domain.
We found that the mutations of tryptophan residues in the N-terminal domain led to expression of inactive LPL. In the case of W114A, only low amounts of LPL monomers were expressed. The absence of activity of the W55A mutant is probably because this residue is in direct vicinity of the catalytic site, and, according to the model of LPL, it is involved in the formation of the oxyanion hole (3). The W114A is predicted to be part of an exposed helix, spanning residues 95-115. In this location, this residue is not likely to be involved in direct interactions at the catalytic site, but it could be necessary for the proper folding or for subunit interaction. The heparin affinity of the N-terminal mutants was decreased and comparable with the wild-type monomer. Previous experiences have shown that the conformation of active dimeric LPL is sensitive to changes in the N-terminal domain, and mutations here usually lead to expression of inactive monomeric enzyme (23).
Heparin-Sepharose chromatography of the C-terminal substitution mutants showed that all mutants were able to form active dimers, although the monomer/dimer ratio varied and was slightly lower for all than for wild-type LPL except for W382A. The reactivity with the 5D2 monoclonal antibody was significantly decreased for all C-terminal mutants, with the exception of W382A. This may indicate that these exchanges cause small local conformational changes that are propagated into the 5D2 epitope, or that the mapped epitope on LPL for 5D2 (residues 396-405) is longer than previously reported (21).
Assays with long-chain triacylglycerols as substrate (IntralipidR or chylomicrons) revealed that mutants W390A, W393A, and W394A and the double mutant W393A/W394A had significantly lower specific catalytic activities than the wild-type LPL. The double mutant exhibited the lowest activity, with only 6% left as compared with wild-type LPL. The activities obtained with rat chylomicrons, the physiological substrate of LPL, were in agreement with those obtained with the synthetic emulsion IntralipidR. Overall, the specific activities for chylomicrons were about 2-fold higher than those for IntralipidR, demonstrating that chylomicrons are more efficient as substrate. One explanation for this could be that the synthetic emulsion may contain excess phospholipids in the form of liposomes, which can compete for binding of LPL (26).
The relative activity of the W382A mutant as compared with wild-type LPL was in the same range for IntralipidR as for tributyrylglycerol, suggesting that Trp382 is not crucial in binding/docking to a lipid/water interface. The mutants W390A, W393A, and W394A and the double mutant W393A/W394A had considerably higher relative activity against tributyrylglycerol than against IntralipidR and chylomicrons, demonstrating that these exchanges severely affected the binding/docking of LPL to emulsions of long-chain triacylglycerols. In all cases more than 70% of the activity remained against tributyrylglycerol, indicating that all the mutants retained most of their active site function.
The increased specific activities against tributyrylglycerol under aggregating conditions (2.5 mM), as compared with activities under water-soluble conditions (0.09 mM), presumably demonstrate the effect of interfacial activation on the catalytic activity (22). The enhancement was about 10-fold for wild-type LPL and for all mutants, except for the double mutant. This indicates that the double mutant has reduced ability to benefit from interaction with a water/lipid interface. A similar tendency was also seen for the W394A mutant.
The calculated apparent Km values of the LPL variants with the long-chain lipid substrates showed that the values were increased for all mutants as compared with wild-type LPL. While for IntralipidR the apparent decrease in lipid affinity was moderate for all mutants, the apparent affinities for chylomicrons were lowered severalfold by replacement of the tryptophans. Our data indicate that binding/docking to chylomicrons is more difficult than binding to IntralipidR. The moderate decrease in apparent lipid affinity (4-8-fold) compared with the marked decrease in maximal specific activities under saturating conditions for both IntralipidR and for chylomicrons suggests that the major effect of the tryptophan substitutions was on the productive orientation of the lipase at the lipid/water interface rather than on the binding/docking to the interface. Williams et al. (8) found that replacement of Trp393 or Trp394 by Ala in C-terminal fragments of LPL spanning residues 313-348 completely abolished the ability of the fragments to bind to lipoproteins.
LPL is a member of the mammalian lipase gene family, consisting also of
pancreatic lipase (PL) and hepatic lipase (HL) (1, 3). We compared the
location of the tryptophans in the C-terminal domain of PL and HL with
that of LPL (Fig. 4). This revealed that the tryptophan
cluster involving residues 390-394 in LPL is not conserved in PL.
Interestingly, the crystallographic studies of van Tilbeurgh et
al. (27) on PL and colipase in the presence of lipid micelles
showed that binding to lipid was mediated by the lid structure together
with hydrophobic regions of colipase. The C-terminal region in PL,
corresponding to residues 390-394 in LPL, interacted with colipase and
not directly with the lipid. Inspection of the C-terminal domain of HL
revealed that residues Trp393 and Trp394 were
not conserved, while tryptophans corresponding to Trp382
and Trp390 in LPL were also present in HL. In HL there is
an insertion of 8 amino acid residues between residue
Ser385 and Asp386 in LPL. This inserted part
contains 2 tryptophan residues. Interestingly, activity against
triolein for a chimera consisting of the N-terminal domain of LPL
(residues 1-335) and the C-terminal domain of HL (residues 330-476)
was significantly lower than activity of wild type LPL (28),
demonstrating the importance of the C-terminal domain for activity
toward insoluble substrates. For both HL and for LPL, the positions of
the tryptophan residues in this region of the C-terminal domain are
conserved between different animal species. The apparent importance of
the specific orientation of the LPL protein at the lipid/water
interface implies that the C-terminal domain, aside from binding to
lipid, may also influence substrate specificity. This was supported by
a study of Davis et al. (29), who showed that exchanging the
C-terminal domains of LPL and of HL resulted in altered specificities
for phospholipids. Dugi et al. (30) have shown that the same
applies also to differences in the lid structure of the N-terminal
domain.
The effect of replacements for tryptophan on the lipid binding and catalytic properties of LPL were more prominent than effects of the same on the ability of LPL to mediate uptake of lipoproteins in cells, presumably via LRP. In a study by Krapp et al. (20), mutants W390A and W393A were shown to be as effective as wild-type LPL. This was also the case for a mutant in which residues 392 and 393 were replaced by those in HL. A reduced ability was shown only for an LPL variant in which residues 390-393 were deleted, but the deletion may have caused also conformational changes affecting other regions.
In summary we conclude that tryptophan residues in the C-terminal domain of LPL are involved in the binding of the enzyme to lipid/water interfaces. We postulate that, in addition to binding, the C-terminal domain is important for the specific orientation of the lipase at the interface and that this orientation may influence both the catalytic efficiency and the substrate specificity.
We are grateful for the expert technical assistance of Ann Sofie Jakobsson and Elianne Appelman.