Aromatic residues in the C-terminal helix of human apoC-I mediate phospholipid interactions and particle morphology.
Human apolipoprotein C-I (apoC-I) is an exchangeable apolipoprotein that binds to lipoprotein particles in vivo. In this study, we employed a LC-MS/MS assay to demonstrate that residues 38-51 of apoC-I are significantly protected from proteolysis in the presence of 1,2-dimyristoyl-3-sn-glycero-phosphocholine (DMPC). This suggests that the key lipid-binding determinants of apoC-I are located in the C-terminal region, which includes F42 and F46. To test this, we generated site-directed mutants substituting F42 and F46 for glycine or alanine. In contrast to wild-type apoC-I (WT), which binds DMPC vesicles with an apparent Kd [Kd(app)] of 0.89 microM, apoC-I(F42A) and apoC-I(F46A) possess 2-fold weaker affinities for DMPC with Kd(app) of 1.52 microM and 1.58 microM, respectively. However, apoC-I(F46G), apoC-I(F42A/F46A), apoC-I(F42G), and apoC-I(F42G/F46G) bind significantly weaker to DMPC with Kd(app) of 2.24 microM, 3.07 microM, 4.24 microM, and 10.1 microM, respectively. Sedimentation velocity studies subsequently show that the protein/DMPC complexes formed by these apoC-I mutants sediment at 6.5S, 6.7S, 6.5S, and 8.0S, respectively. This is compared with 5.0S for WT apoC-I, suggesting the shape of the particles was different. Transmission electron microscopy confirmed this assertion, demonstrating that WT forms discoidal complexes with a length-to-width ratio of 2.57, compared with 1.92, 2.01, 2.16, and 1.75 for apoC-I(F42G), apoC-I(F46G), apoC-I(F42A/F46A), and apoC-I(F42G/F46G), respectively. Our study demonstrates that the C-terminal amphipathic alpha-helix of human apoC-I contains the major lipid-binding determinants, including important aromatic residues F42 and F46, which we show play a critical role in stabilizing the structure of apoC-I, mediating phospholipid interactions, and promoting discoidal particle morphology.
James
Patrick F
PF
Department of Biochemistry and Molecular Biology, University of Melbourne, Parkville, Victoria 3010, Australia.
Dogovski
Con
C
Dobson
Renwick C J
RC
Bailey
Michael F
MF
Goldie
Kenneth N
KN
Karas
John A
JA
Scanlon
Denis B
DB
O'Hair
Richard A J
RA
Perugini
Matthew A
MA
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry* / Q000378:metabolism; D053302:Apolipoprotein C-I / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002853:Chromatography, Liquid; D002942:Circular Dichroism; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry / Q000378:metabolism; D005998:Glycine / Q000235:genetics / Q000378:metabolism; D006801:Humans; D050356:Lipid Metabolism; D008055:Lipids / Q000737:chemistry; D008074:Lipoproteins / Q000378:metabolism / Q000648:ultrastructure; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010316:Particle Size; D010743:Phospholipids / Q000737:chemistry / Q000378:metabolism*; D017433:Protein Structure, Secondary; D053719:Tandem Mass Spectrometry
C505803:APOC1 protein, human; D000596:Amino Acids; D053302:Apolipoprotein C-I; D008055:Lipids; D008074:Lipoproteins; D010743:Phospholipids; D005998:Glycine; D004134:Dimyristoylphosphatidylcholine