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 o f high density lipoprotein (HDL) cholesteryl eater without the uptake and d egradation of the particle. In transfected cells SR-BI recognizes HDL, low density lipoprotein (LDL) and modified LDL, protein-free lipid vesicles con taining anionic phospholipids, and recombinant lipoproteins containing apol ipoprotein (apo) A-I, apoA-II, apoE, or apoCIII, The molecular basis for th e recognition of such diverse ligands by SR-BI is unknown. We have used dir ect 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, HDL3, or in a lipid-free state binds to mSR-BI with high affinity (K-d congruent to 5-8 mu g/ml). ApoA-I in each of these forms was efficiently cross-linked to cell surface mSR-BI, indicating that direct protein-protein contacts are t he predominant feature that drives the interaction between HDL and mSR-BI, When complexed with dimyristoylphosphatidylcholine, the N-terminal and C-te rminal CNBr fragments of apoA-I each bound to SR-BI in a saturable, high af finity manner, and each cross-linked efficiently to mSR-BI. Thus, mSR-BI re cognizes 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 amphi pathic cw-helical repeat units of apoA-I, This hypothesis explains the inte raction of SR-BI with a wide variety of apolipoproteins via a specific seco ndary structure, the class A amphipathic alpha-helix, that is a common stru ctural motif in the apolipoproteins of HDL, as well as LDL.