Contributions of a disulfide bond and a reduced cysteine side chain to the intrinsic activity of the HDL receptor SR-BI

Abstract

The high density lipoprotein (HDL) receptor, scavenger receptor class B, type I (SR-BI), binds HDL and mediates selective cholesteryl ester uptake. SR-BI's structure and mechanism are poorly understood. We used mass spectrometry to assign the two disulfide bonds in SR-BI that connect cysteines within the conserved Cys[superscript 321]-Pro[superscript 322]-Cys[superscript 323] (CPC) motif and connect Cys[superscript 280] to Cys[superscript 334]. We used site-specific mutagenesis to evaluate the contributions of the CPC motif and the side chain of extracellular Cys[superscript 384] to HDL binding and lipid uptake. The effects of CPC mutations on activity were context dependent. Full wild-type (WT) activity required Pro[superscript 322] and Cys[superscript 323] only when Cys[superscript 321] was present. Reduced intrinsic activities were observed for CXC and CPX, but not XXC, XPX or XXX mutants (X≠WT residue). Apparently, a free thiol side chain at position 321 that cannot form an intra-CPC disulfide bond with Cys[superscript 323] is deleterious, perhaps because of aberrant disulfide bond formation. Pro[superscript 322] may stabilize an otherwise strained CPC disulfide bond, thus supporting WT activity, but this disulfide bond is not absolutely required for activity. C[superscript 384]X (X=S,T,L,Y,G,A) mutants exhibited altered activities that varied with the side chain's size: larger side chains phenocopied WT SR-BI treated with its thiosemicarbazone inhibitor BLT-1 (increased binding, decreased uptake); smaller side chains produced almost inverse effects (increased uptake:binding ratio). C[superscript 384]X mutants were BLT-1 resistant, supporting the proposal that Cys[superscript 384]'s thiol interacts with BLT-1. We discuss the implications of our findings on the functions of the extracellular loop cysteines in SR-BI and compare our results to those presented by other laboratories.