Binding and cross-linking studies show that scavenger receptor BI interacts with multiple sites in apolipoprotein A-I and identify the class A amphipathic alpha-helix as a recognition motif.
Scavenger receptor, class B, type I (SR-BI) mediates the selective uptake of high density lipoprotein (HDL) cholesteryl ester without the uptake and degradation of the particle. In transfected cells SR-BI recognizes HDL, low density lipoprotein (LDL) and modified LDL, protein-free lipid vesicles containing anionic phospholipids, and recombinant lipoproteins containing apolipoprotein (apo) A-I, apoA-II, apoE, or apoCIII. The molecular basis for the recognition of such diverse ligands by SR-BI is unknown. We have used direct binding analysis and chemical cross-linking to examine the interaction of murine (m) SR-BI with apoA-I, the major protein of HDL. The results show that apoA-I in apoA-I/palmitoyl-oleoylphosphatidylcholine discs, HDL(3), or in a lipid-free state binds to mSR-BI with high affinity (K(d) congruent with 5-8 microgram/ml). ApoA-I in each of these forms was efficiently cross-linked to cell surface mSR-BI, indicating that direct protein-protein contacts are the predominant feature that drives the interaction between HDL and mSR-BI. When complexed with dimyristoylphosphatidylcholine, the N-terminal and C-terminal CNBr fragments of apoA-I each bound to SR-BI in a saturable, high affinity manner, and each cross-linked efficiently to mSR-BI. Thus, mSR-BI recognizes multiple sites in apoA-I. A model class A amphipathic alpha-helix, 37pA, also showed high affinity binding and cross-linking to mSR-BI. These studies identify the amphipathic alpha-helix as a recognition motif for SR-BI and lead to the hypothesis that mSR-BI interacts with HDL via the amphipathic alpha-helical repeat units of apoA-I. This hypothesis explains the interaction of SR-BI with a wide variety of apolipoproteins via a specific secondary structure, the class A amphipathic alpha-helix, that is a common structural motif in the apolipoproteins of HDL, as well as LDL.
Williams
D L
DL
Department of Pharmacological Sciences, University Medical Center, State University of New York, Stony Brook, New York 11794, USA. dave@pharm.sunysb.edu
de La Llera-Moya
M
M
Thuahnai
S T
ST
Lund-Katz
S
S
Connelly
M A
MA
Azhar
S
S
Anantharamaiah
G M
GM
Phillips
M C
MC
D020816:Amino Acid Motifs; D000818:Animals; D016632:Apolipoprotein A-I / Q000378:metabolism*; D001665:Binding Sites; D018955:CD36 Antigens / Q000737:chemistry / Q000378:metabolism*; D019556:COS Cells; D003432:Cross-Linking Reagents; D006801:Humans; D008565:Membrane Proteins; D011485:Protein Binding; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; D051117:Scavenger Receptors, Class A; D051122:Scavenger Receptors, Class B
D016632:Apolipoprotein A-I; D018955:CD36 Antigens; D003432:Cross-Linking Reagents; D008565:Membrane Proteins; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; C495124:SCARB1 protein, human; C454739:Scarb1 protein, mouse; D051117:Scavenger Receptors, Class A; D051122:Scavenger Receptors, Class B