From the Lipid and the § Lipoprotein
Research Group, Protein Engineering Network of Centres of
Excellence, Department of Biochemistry, University of Alberta,
Edmonton, Alberta T6G 2S2, Canada
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ABSTRACT |
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Apolipophorin III (apoLp-III) from
the Sphinx moth, Manduca sexta, is an 18-kDa exchangeable
apolipoprotein that reversibly associates with lipoprotein particles.
In the absence of lipid, apoLp-III exists as an elongated bundle of
five amphipathic -helices. Upon lipid association, the protein is
postulated to undergo a major conformational change, wherein the bundle
opens around hinge loop regions, resulting in exposure of its
hydrophobic interior. Fluorescence quenching techniques have been
employed to study apoLp-III helix topography and spatial arrangement in
phospholipid disc complexes and intact lipoprotein particles. Intrinsic
fluorescence of the single tyrosine in apoLp-III was exploited to
monitor the location of helix 5 in model disc complexes. To investigate
other regions of the protein, site-directed mutagenesis was performed to introduce cysteine residues, replacing Asn-40 (helix 2, N40C) or
Leu-90 (helix 3, L90C), thereby providing two mutant apoLp-IIIs, each
with a single site for covalent attachment of the extrinsic fluorescent
probe, N-(1-pyrene) maleimide. In the lipid-free state, pyrene-N40C- and pyrene-L90C-apoLp-III were highly accessible to the
negatively charged aqueous quencher KI, yielding
Ksv values of 27.1 and 19.8 M
1, respectively. Upon binding to the surface
of a spherical lipoprotein particle, Ksv values
for KI decreased by about 90% for both pyrene-labeled apoLp-IIIs,
indicating a significant change in the local microenvironment of the
fluorophores. A lesser decrease in Ksv was
observed when the pyrene-labeled apoLp-IIIs were bound to phospholipid
disc complexes. When spin-labeled fatty acids 5-doxylstearic acid and 12-doxylstearic acid were used as lipophilic quenchers, tyrosine and
pyrene fluorescence were more effectively quenched by 5-doxylstearic acid in both phospholipid bilayer disc complexes and spherical lipoprotein particles. These data provide insight into the spatial topography of apoLp-III
-helices in phospholipid disc complexes and
support the concept that interaction with spherical lipoprotein particles results in superficial contact of apoLp-III helical segments
with the monolayer surface, providing a basis for its reversible
binding ability.
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INTRODUCTION |
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Plasma lipoprotein metabolism is regulated to a large extent by
reversible binding of exchangeable apolipoproteins (1, 2). These
amphipathic -helical proteins have the ability to exist in both
water-soluble and lipid-bound forms, a property which is the basis of
their physiological role in lipoprotein metabolism. Apolipophorin III
(apoLp-III)1 from the Sphinx
moth, Manduca sexta, is a 166-amino acid member of the
exchangeable apolipoprotein family that has been well characterized (3). Structural information is available for the lipid-free form of
M. sexta apoLp-III (4), as well as Locusta
migratoria apoLp-III (5). Both proteins exist as globular
up-and-down amphipathic
-helix bundles, with their polar and charged
residues facing the aqueous environment and their hydrophobic residues oriented toward the protein interior. Several lines of evidence support
the view that lipid interaction of apoLp-III requires opening of the
bundle, presumably around the hinge loop regions located between
helices 2 and 3 and between helices 4 and 5 (5, 6). This conformational
change would lead to exposure of the hydrophobic interior of the
protein, facilitating lipid association. In this model, helix-helix
interactions that maintain the structure of the bundle
conformation2 are replaced by
helix-lipid interactions. Chemical cross-linking and electron
microscopy studies (8) and Fourier transform infrared spectroscopy (9)
of apoLp-III in phospholipid bilayer discs indicate that the protein
adopts an open state, oriented peripherally, surrounding the bilayer
disc with its helices aligned perpendicular to the phospholipid fatty
acyl chains. Although we have a good global view of how apoLp-III
interacts with various lipid systems, key issues remain unresolved,
including the molecular basis of conformational opening and the
structure of the lipid-bound form.
To further understand the lipid-bound conformation of apoLp-III, we
employed fluorescence techniques to study the accessibility to
quenchers of the single tyrosine, as well as extrinsic fluorophores covalently bound to cysteine residues introduced at specified locations
by site-directed mutagenesis. Although fluorescence from Tyr-145
(located in helix 5) is negligible in the lipid-free state (10, 11), a
significant enhancement was observed upon lipid binding (8), which
facilitated quenching studies. By comparison of the relative
effectiveness of different lipid-based quenchers, information about the
spatial alignment of apoLp-III -helices complexed with phospholipids
and spherical lipoprotein surfaces was obtained.
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EXPERIMENTAL PROCEDURES |
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Materials--
2,2-dithiodipyridine (AldrithiolTM),
5-doxylstearic acid, and 12-doxylstearic acid spin labels were obtained
from Aldrich. N-(1-Pyrene)maleimide was purchased from
Molecular Probes, Inc. (Eugene, OR). Dimyristoylphosphatidylcholine,
phospholipase C from Bacillus cereus, and trifluoroethanol
were purchased from Sigma, Sepharose CL-6B column matrix from Pharmacia
Biotech Inc., and pET expression vector from Novagen (Madison, WI). All
other chemicals and solvents were of analytical grade.
Site-directed Mutagenesis and Bacterial Expression-- Site-directed mutagenesis of apoLp-III was performed as described earlier, using mismatched oligonucleotide primers (6). In one mutation, Asn-40 at the N terminus of helix 2 (close to the polar/nonpolar interface) was replaced by Cys (N40C). In another experiment, Leu-90 at the C terminus of helix 3 (in the center of the nonpolar face of the amphipathic helix) was replaced by Cys (L90C) (4). The genes encoding full-length apoLp-III bearing the mutations were cloned into the pET vector and sequenced by the dideoxynucleotide chain termination method on the double-stranded template (12). The apoLp-III mutants were expressed in Escherichia coli BL21 cells as described by Ryan et al. (13).
Purification and Labeling of Recombinant
apoLp-III--
Wild-type, N40C-, and L90C-apoLp-III were purified by
reversed phase high performance liquid chromatography as described
previously (6). Electrospray mass spectrometry yielded a mass of
18,370 ± 1.5 and 18,371 ± 1.5 Da for N40C-apoLp-III and
L90C-apoLp-III, respectively, as compared with 18,381 Da for wild-type
apoLp-III (VG quattro electrospray mass spectrometer, Fisons
Instruments, Manchester, UK). When the cysteine-containing mutant
apoLp-IIIs were labeled with 2,2-dithiodipyridine, a molecular mass
increment of 108 daltons was observed. To label the cysteine mutants
with N-(1-pyrene)maleimide, the probe was solubilized in
dimethyl sulfoxide and incubated in a 2:1 molar ratio with either N40C-
or L90C-apoLp-III (in 100 mM sodium phosphate, pH 7.5) for
2.5 h at 37 °C in the dark. Unbound pyrene was separated from
labeled protein by gel filtration chromatography. The protein was
analyzed by SDS-polyacrylamide gel electrophoresis to verify labeling
and visualized under UV light.
Structural and Functional Characterization of Pyrene-labeled N40C- and L90C-apoLp-III-- Circular dichroism spectra of pyrene-N40C-apoLp-III and L90C-apoLp-III (in 50 mM sodium phosphate, pH 7.0) were collected on a Jasco J-720 spectropolarimeter at 25 °C as described previously (13). The functional properties of pyrene-N40C- and pyrene-L90C-apoLp-III were characterized in a lipoprotein binding assay as described elsewhere (6).
Preparation of apoLp-III·DMPC Complexes-- Wild-type apoLp-III, pyrene-N40C-apoLp-III, or pyrene-L90C-apoLp-III were complexed with DMPC as described by Weintzek et al. (8). The final lipid:protein molar ratio was ~150:1.
Preparation of Lipoprotein-associated apoLp-III-- Lipoprotein-associated wild-type, pyrene-N40C-, or pyrene-L90C-apoLp-III was prepared according to Liu et al. (14). Wild-type, pyrene-N40C-, or pyrene-L90C-apoLp-III (1.5 mg) was incubated with human low density lipoprotein (LDL) (2 mg protein) in the presence of 6 units of phospholipase C (from B. cereus) for 30 min at 37 °C. Unbound protein was separated by Sepharose CL-6B gel filtration chromatography.
Fluorescence Spectroscopy--
Fluorescence measurements were
performed on a Perkin-Elmer MPF-44B spectrofluorometer with the
temperature maintained at 20 °C in a thermostatted cell holder. A
bandwidth of 4 nm was used for both excitation and emission
monochromators. Excitation and emission spectra of pyrene-labeled
apoLp-IIIs were recorded by setting the emission and excitation
wavelengths at 375 and 345 nm, respectively. Tyrosine fluorescence in
wild-type apoLp-III (which lacks tryptophan) was measured at excitation
and emission wavelengths of 277 and 300 nm, respectively. To measure
the effect of aqueous quenchers, lipid-free pyrene-labeled apoLp-III
and DMPC-bound or LDL-bound pyrene-N40C- and pyrene-L90C-apoLp-III were
treated with increasing concentrations of potassium iodide in 50 mM sodium phosphate, pH 7.5. KI solutions contained 1 mM sodium thiosulfate to suppress free iodine formation.
Similarly, for depth-dependent quenching studies, aliquots
of 5-doxylstearic acid or 12-doxylstearic acid (5-DSA and 12-DSA,
respectively (1.3 mM stock in ethanol)) were added to
lipid-bound samples (final concentration of ethanol, 1% by volume),
and fluorescence intensities were measured following equilibration for
5 min. The effective quenching constants were determined according to
the Stern-Volmer equation, Fo/F = 1 + Ksv[Q], where
Fo and F are fluorescence intensities
in the absence and presence of quencher, respectively, and
[Q] is the concentration of quencher. The slope of the
plot of Fo/F versus [Q]
is the effective Stern-Volmer quenching constant,
Ksv (15). When using spin-labeled fatty acids as
quenchers, the classic Stern-Volmer equation for free diffusion no
longer applies because the quencher is not uniformly distributed over
the corresponding space coordinate (16, 17). Therefore, for purposes of
relative comparison within spin-labeled fatty acids, we have calculated
an apparent quenching constant (Kapp) in place
of Ksv.
Analytical Procedures-- SDS-polyacrylamide gel electrophoresis was performed on 12% acrylamide slab gels run at 30 mA constant current and stained with Coomassie Brilliant Blue R-250. Protein concentrations were determined by the bicinchoninic acid assay (Pierce), and phosphatidylcholine was determined using an enzyme-based colorimetric assay (Wako Pure Chemical Industries, Ltd., Osaka, Japan).
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RESULTS |
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Structural and Functional Characterization of Alkylated N40C- and
L90C-apoLp-III--
Modification of N40C- and L90C-apoLp-IIIs by
covalent attachment of pyrene maleimide resulted in labeling of the
single engineered cysteine residue. The presence of a single attachment
site on either protein and the availability of the proteins for
alkylation were examined by two independent methods. The first method
used 2,2-dithiodipyridine alkylation, which under native conditions resulted in covalent linkage of the thiopyridyl moiety to the cysteine,
yielding a product with a molecular mass increment of 108 Da. The
second method employed pyrene maleimide, which upon covalent linkage to
the free sulfhydryl yielded a product with a molecular mass increment
of 297 Da (from 18,371 to 18,668 Da). These data indicate that there is
only one cysteine present in both recombinant proteins and that, under
the conditions studied, they are not involved in intermolecular
disulfide bond formation. N-(1-Pyrene)maleimide has the
unique property of becoming fluorescent only after covalent attachment
to a sulfhydryl group. Following SDS-polyacrylamide gel
electrophoresis, N40C-apoLp-III and pyrene-L90C-apoLp-III were
fluorescent when viewed under a UV illuminator. Fluorescence excitation
and emission spectra of pyrene-N40C- and L90C-apoLp-III are shown in
Fig. 1. Typical fluorescence properties
of pyrene, such as excitation at 335 and 345 nm and emission at 375 and
396 nm, with a high quantum yield, are evident, confirming labeling of
the cysteine residue on the proteins. Circular dichroism spectra revealed that the secondary structure content of pyrene-N40C- and
pyrene-L90C-apoLp-III (60%
-helix) is very similar to that of
wild-type apoLp-III (10).
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Potassium Iodide Quenching--
The aqueous quencher, KI, was used
to probe the accessibility and alteration in the local microenvironment
of pyrene-labeled apoLp-IIIs in lipid-free and lipid-bound states (19).
Table I shows the effective Stern-Volmer
quenching constants (Ksv) obtained for
pyrene-N40C- and pyrene-L90C-apoLp-III. Iodide, which quenches by a
dynamic, collisional mechanism, was an efficient quencher of
pyrene-apoLp-III fluorescence in the absence of lipid. When
pyrene-labeled N40C-apoLp-III was unfolded in 2 M guanidine HCl, the observed Ksv (30.5 M1) was similar to that for the native
protein, indicating that the pyrene moiety in native N40C-apoLp-III is
highly accessible to KI. With pyrene-N40C-apoLp-III·DMPC complexes,
the extent of quenching by iodide was similar to that in the lipid-free
state. On the other hand, with pyrene-L90C-apoLp-III·DMPC complexes, Ksv decreased by nearly 40% compared with the
lipid-free state. This reduced quenching efficiency may be due to a
change in the microenvironment of the fluorophore upon interaction with
the disc particles. Alternatively, it may be that the hydrophobic pyrene molecule is buried in the interior of the phospholipid bilayer,
thereby denying access to iodide. Iodide was less effective at
quenching pyrene-N40C- or pyrene-L90C-apoLp-III fluorescence when bound
to the surface of LDL particles. The observed significant reduction in
Ksv values indicates sequestration of the
fluorophore in both labeled proteins away from the aqueous
environment.
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Lipophilic Quenchers-- Spin-labeled fatty acids, with the doxyl moiety located at different positions along the hydrocarbon backbone of the fatty acyl chain, serve as useful quenchers that operate within a lipid environment (20). These quenchers may be used as "molecular rulers" to characterize the interaction of a fluorophore with a given lipid surface. 5-DSA and 12-DSA were incorporated into lipid particles and used to estimate the spatial disposition of Tyr-145 in wild-type apoLp-III and the pyrene moiety on N40C- and L90C-apoLp-III, respectively. Upon binding DMPC, the globular apoLp-III helix bundle undergoes a conformational opening, aligning its helical segments around the perimeter of the disc complexes (8, 9). Quenching of tyrosine and pyrene fluorescence upon binding of apoLp-III to DMPC bilayer discs is shown in Figs. 3 and 4, respectively. A plot of Fo/F versus [Q] yields a straight line, the slope of which is equivalent to the apparent quenching constant, Kapp (inset in Figs. 3 and 4). Table II shows that 5-DSA quenched tyrosine fluorescence to a significantly larger extent than 12-DSA. Similarly, in both pyrene-N40C-apoLp-III/DMPC and pyrene-L90C-apoLp-III·DMPC complexes, a greater extent of quenching was observed with 5-DSA.
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DISCUSSION |
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The helix-bundle molecular architecture is a common structural motif of exchangeable apolipoproteins in their lipid-free state (4, 5, 23). Such an organization facilitates its existence as a soluble protein in an aqueous medium, such as plasma, and permits binding to lipid surfaces through a conformational change. To obtain information about lipid-associated conformations of apoLp-III, the fluorescence properties of its unique tyrosine residue (located close to the center of the nonpolar face at position 145 in helix 5) have been investigated (8, 10, 11, 24). In these studies, Tyr-145 served as a reporter of the conformational status of helix 5 in different environments. However, examining intrinsic tyrosine fluorescence has limited applications for the following reasons: 1) its quantum yield is low, 2) it cannot be selectively evaluated in the presence of other proteins, and 3) the focus is limited to helix 5. To circumvent these problems and expand the versatility of the fluorescence approach to investigating structural aspects of lipid-associated apoLp-III, we have introduced single cysteine residues into the protein by site-directed mutagenesis at Asn-40 (helix 2) or Leu-90 (helix 3). Following covalent attachment of pyrene to the free sulfhydryl, fluorescence quenching was performed to evaluate the orientation of apoLp-III helical segments in DMPC disc complexes and the depth of penetration of the helices into the monolayer surface of intact spherical lipoprotein particles. Advantages of this approach include the high quantum yield of the pyrene fluorophore, the lack of interference from other proteins that may be present in the system under study, and the ability to introduce the reporter fluorophore at specified locations in the protein.
Changes in accessibility to quenching by KI were measured to monitor alterations in the local microenvironment of the labeled cysteine residues. In the lipid-free globular form, high quenching constants were obtained for both pyrene-N40C- and pyrene-L90C-apoLp-III. This indicates that the pyrene moiety is readily accessible to the negatively charged iodide. Examination of the structure of lipid-free apoLp-III (4) revealed several lysine residues in helices 3 and 4 in close proximity to N40C, as well as at positions i, i + 1 and i, i + 4 in helix 2. Similarly, lysines were found in helix 4, as well as at the i, i + 2 position to L90C in helix 3. Thus, a positively charged microenvironment in the vicinity of the fluorophore likely facilitates iodide quenching. Upon binding to model disc complexes, a 40% decrease in Ksv was noted for pyrene-L90C-apoLp-III, yet little or no change in Ksv was observed for pyrene-N40C-apoLp-III in these complexes. This result may be explained if helix 2 is located closer to the quarternary amine of choline head groups of the phospholipids, promoting iodide quenching (25). When bound to the surface of spherical lipoprotein particles, Ksv for both pyrene-labeled apoLp-IIIs decreased by about 90% compared with the lipid-free state. A likely explanation for this large decrease in Ksv is burial of the hydrophobic pyrene moiety in the lipoprotein monolayer, which denies access to the hydrophilic iodide. Taken together, these results document clear differences in the microenvironment of the probe upon association with phospholipid disc complexes versus lipoprotein particles, indicating that these represent distinct lipid surfaces for binding of exchangeable apolipoproteins. Furthermore, apolipoprotein/lipoprotein complexes created using phospholipase C-modified LDL provide a useful, structurally relevant alternative to phospholipid discs for studying the properties of lipid-associated exchangeable apolipoproteins. Indeed, these particles are amenable to studies of the depth of penetration of apolipoprotein helical segments into the lipoprotein surface.
Use of Fatty Acid Spin Labels to Study Lipid-bound Protein Conformations-- Protein-lipid interactions in model membranes have been studied in detail using spin-labeled fatty acids (26). Fatty acid spin labels quench via a collisional mechanism that is short-range in nature. Thus, they have to be present in or near the sphere of action of the fluorophore to quench effectively, with an optimal interaction distance of 4-6 Å (27). We have estimated the transverse disposition and depth of penetration of apoLp-III helices in model disc complexes and lipoprotein particles, respectively, based on the parallax fluorescent quenching method originally proposed by Chattopadhyay and London (28). This method recommends the use of spin-labeled phospholipids instead of spin-labeled fatty acids due to the vertical fluctuations and relatively faster lateral mobility of the latter (29). However, our attempts to make discs containing spin-labeled phospholipids were unsuccessful as judged by electron microscopic characterization of the product particles. The choice to use spin-labeled fatty acids in our experiments was to our advantage because they can be added directly to the lipoprotein or disc system with bound apoLp-III. Quenching by spin-labeled fatty acids has been successfully used in several cases to measure the location of protein-bound fluorophores in membranes by comparison of the quenching efficiency of probes placed at various depths in the membrane (30-32). In 5-DSA, the doxyl moiety is located approximately 6 Å from the membrane/water interface, whereas in 12-DSA, it is located about 15 Å from the interface, deep in the membrane interior (20).
Fluorescence Quenching of apoLp-III-phospholipid Disc Particles-- Wild-type apoLp-III contains a single tyrosine residue, which is an intrinsic fluorophore. Previous studies have shown that helix 5, in isolation, cannot form complexes with phospholipid in vitro (11). However, quenching of its fluorescence by spin-labeled fatty acids when helix 5 is present in the intact protein indicates that despite its terminal location in the protein sequence, helix 5 contacts the lipid surface in a stable, oriented manner. Pyrene-labeling of two cysteine mutant apoLp-IIIs allowed further quenching studies, yielding information on the disposition of helices 2 and 3, as well. In the lipid-free protein, NMR structural studies revealed that helix 5 interacts with helices 1 and 2, whereas there are no contacts between helices 1 and 2 themselves (4). In studies of the conformation of apoLp-III in phospholipid disc particles, cross-linking experiments indicate that about six molecules of apoLp-III align themselves around the circumference of the disc complexes (8). On the basis of Fourier transform infrared spectroscopy studies, it is concluded that the helices adopt a unique orientation, with their axes oriented perpendicular to the fatty acyl chains of the phospholipid bilayer (9). If conformational opening occurs as proposed, both ends of the molecule should contact the lipid surface. The quenching studies with pyrene-N40C- and pyrene-L90C-apoLp-III support this view. Furthermore, because 5-DSA displayed a more pronounced quenching efficiency than 12-DSA in the disc complexes, the helices containing Tyr-145 (helix 5), pyrene-N40C (helix 2), and pyrene-L90C (helix 3) likely align closer to the water/lipid interface. These data suggest that helix alignment in these complexes is not random but rather that the protein adopts a preferred orientation with respect to the bilayer. Further studies will be required to delineate the precise location of the different helices with respect to each other and to the phospholipid bilayer.
Superficial Association of apoLp-III with Lipoprotein
Surfaces--
All exchangeable apolipoproteins identified to date
share a common amphipathic -helix structural motif (33). These
helices have well-defined polar and nonpolar faces that can interact
with the aqueous and hydrophobic milieu, respectively, on a lipoprotein surface. This feature imparts to exchangeable apolipoproteins the
structural adaptability to reversibly associate with circulating lipoproteins. We have examined the association of apoLp-III with lipoprotein surfaces by first allowing the pyrene-labeled protein to
bind to the surface of LDL particles modified by treatment with
phospholipase C, a situation that mimics the physiological setting
wherein apoLp-III binds to lipophorin particles in response to
diacylglycerol enrichment (34). We have estimated the location of the
fluorophore within the lipoprotein surface monolayer by comparing the
quenching obtained with spin labels at two different depths. With
pyrene-N40C-apoLp-III or pyrene-L90C-apoLp-III, the fluorophore was
quenched 6-10-fold more with 5-DSA than with 12-DSA. Thus, it is
estimated that the helices are located near the water/lipid interface
with their axes perpendicular to the phospholipid monolayer. Based on
the hinged loop opening, it may be envisaged that the hydrophobic
interior of the protein is superficially embedded with its helical axes
parallel to the lipoprotein surface in contact with the
diacylglycerol-containing phospholipid monolayer (22). Because we are
monitoring fluorescence at both ends of the protein (in its open
conformation), this superficial binding is representative of the entire
protein, implying a "loose binding" of apoLp-III with the
lipoprotein surface. The superficial mode of binding of apoLp-III to
the lipoprotein surface is functionally relevant given the reversible
nature of exchangeable apolipoprotein interactions. This would
facilitate dissociation from lipid surfaces without a large input of
energy and is consistent with displacement studies performed by Liu
et al. (35) in which apoLp-III was readily displaced from
the surface of low density lipophorin by human apoA-I. A similar mode
of interaction has been proposed for the binding of class A amphipathic
helices of apolipoproteins with the phospholipid monolayer, although a
deeper penetration of the nonpolar face has been implied (36). However,
in the case of apoLp-III, the binding interaction appears to be
correlated with exposed diacylglycerol, which probably acts as a
trigger for initiating association with the lipoprotein surface.
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ACKNOWLEDGEMENTS |
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We thank Kim Oikawa for performing CD studies and Paul Semchuk for the mass spectrometric analysis.
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FOOTNOTES |
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* This work was supported by grants from the Medical Research Council of Canada (to R. O. R.).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: 328 Heritage Medical Research Centre, University of Alberta, Edmunton, Alberta T6G 252, Canada. Tel.: 403-492-5153; Fax: 403-492-3383; E-mail: Robert.Ryan{at}Ualberta.ca.
1 The abbreviations used are: apoLp-III, apolipophorin III; DMPC, dimyristoylphosphatidylcholine; DSA, doxylstearic acid; LDL, low density lipoprotein; pyrene, N-(1-pyrene)maleimide.
2 Wang, J., Narayanaswami, V., Sykes, B. D., and Ryan, R. O. (1998) Protein Sci., in press.
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REFERENCES |
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