Dimerization of the scavenger receptor class B type I

Scavenger receptor class B, type I (SR-BI), a CD36 superfamily member, is an oligomeric high density lipoprotein (HDL) receptor that mediates negatively cooperative HDL binding and selective lipid uptake. We identified in the N-terminal transmembrane (N-TM) domain of SR-BI a conserved glycine dimerization motif, G 15 X 2 G 18 X 3 AX 2 G 25 , of which the submotif G 18 X 3 AX 2 G 25 significantly contributes to homodimerization and lipid uptake activity. SR-BI variants were generated by mutations (single or multiple Gly 3 Leu substitutions) or by replacing the N-TM domain with those from other CD36 superfamily members containing (croquemort) or lacking (lysosomal integral membrane protein (LIMP) II) this glycine motif (chimeras). None of the SR-BI variants exhibited altered surface expression (based on antibody binding) or HDL binding. However, the G15L/G18L/G25L triple mutant exhibited reductions in cell surface homo-oligomerization (>10-fold) and the rate of selective lipid uptake (ϳ2-fold). Gly 18 and Gly 25 were necessary for normal lipid uptake activity of SR-BI and the SR-BI/ croquemort chimera. The lipid uptake activity of the glycine motif-deficient SR-BI/LIMP II chimera was low but could be increased by introducing glycines at positions 18 and 25. The rate of lipid uptake mediated by SR-BI/LIMP II chimeras was proportional to the extent of receptor oligomerization. Thus, the glycine dimerization motif G 18 X 3 AX 2 G 25 in the N-TM domain of SR-BI contributes substantially to the homo-oligomerization and lipid transport activity of SR-BI but does not influence the negative cooperativity of HDL binding. Oligomerization-independent binding cooperativity suggests that classic allostery is not involved and that the negative cooperativity is probably the consequence of a "lattice effect" (interligand steric interference accompanying binding to adjacent receptors).The high density lipoprotein (HDL) receptor SR-BI 2 is a cell surface receptor expressed in the liver and other tissues that regulates plasma lipoprotein cholesterol levels and metabolism (Refs. 1 and 3-6; for a review, see Ref.2). SR-BI mediates cellular uptake of cholesteryl esters and other lipids from HDL and has been proposed to play a key role in moving cholesterol from peripheral tissues to the liver and then out of the body (Refs. 7-10; for a review, see Ref.2). SR-BI influences HDL metabolism in humans (11,12) in whom the risk of atherosclerotic disease is inversely or directly proportional to plasma concentrations of HDL cholesterol and low density lipoprotein (LDL) cholesterol, respectively (13-15). Studies in mice have shown that SR-BI protects against atherosclerosis and coronary heart disease (Refs. 16 -22; for a review, see Ref.2).SR-BI is a member of the CD36 superfamily of proteins in metazoans (ϳ25-30% sequence identities). Members of this superfamily include lysosomal integral membrane protein (LIMP) II (23) and croquemort (a Drosophila melanogaster hemocyte/macrophage receptor for apoptotic cells) (24). Most CD36 superfamily membe...

Section: Sequence Alignment Of Sr-bi and Cd36 Reveals Existence Of Cosupporting

confidence: 91%

Glycine Dimerization Motif in the N-terminal Transmembrane Domain of the High Density Lipoprotein Receptor SR-BI Required for Normal Receptor Oligomerization and Lipid Transport

Gaidukov

Nager

et al. 2011

“…The reduced V max of the apoA-I Ϫ/Ϫ HDL for HDL CE transfer likely contributes to, but cannot completely explain, this severe reduction in adrenal CE accumulation. Additionally, electron microscopic studies suggest that apoA-I Ϫ/Ϫ HDL are not well retained in the microvillar channels that are believed to be the cell surface compartment in which SR-BI-mediated HDL CEselective uptake occurs (13,51,52). The failure to retain HDL in microvillar channels may also contribute to adrenal CE depletion in the apoA-I Ϫ/Ϫ mouse.…”

Section: Apoa-imentioning

confidence: 99%

Apolipoprotein A-I Is Necessary for thein Vivo Formation of High Density Lipoprotein Competent for Scavenger Receptor BI-mediated Cholesteryl Ester-selective Uptake

Temel¹,

Walzem²,

Banka³

et al. 2002

The severe depletion of cholesteryl ester (CE) in steroidogenic cells of apoA-I ؊/؊ mice suggests that apolipoprotein (apo) A-I plays a specific role in the high density lipoprotein (HDL) CE-selective uptake process mediated by scavenger receptor BI (SR-BI) in vivo. The nature of this role, however, is unclear because a variety of apolipoproteins bind to SR-BI expressed in transfected cells. In this study the role of apoA-I in SR-BImediated HDL CE-selective uptake was tested via analyses of the biochemical properties of apoA-I ؊/؊ HDL and its interaction with SR-BI on adrenocortical cells, hepatoma cells, and cells expressing a transfected SR-BI. apoA-I؊/؊ HDL are large heterogeneous particles with a core consisting predominantly of CE and a surface enriched in phospholipid, free cholesterol, apoA-II, and apoE. Functional analysis showed apoA-I ؊/؊ HDL to bind to SR-BI with the same or higher affinity as compared with apoA-I ؉/؉ HDL, but apoA-I ؊/؊ HDL showed a 2-3-fold decrease in the V max for CE transfer from the HDL particle to adrenal cells. These results indicate that the absence of apoA-I results in HDL particles with a reduced capacity for SR-BI-mediated CE-selective uptake. The reduced V max illustrates that HDL properties necessary for binding to SR-BI are distinct from those properties necessary for the transfer of HDL CE from the core of the HDL particle to the plasma membrane. The reduced V max for HDL CE-selective uptake likely contributes to the severe reduction in CE accumulation in steroidogenic cells of apoA-I ؊/؊ mice.

“…SR-BI is highly expressed in the liver and steroidogenic tissues and expressed to lower levels in the intestine and vasculature in adult mice (1)(2)(3)(4). SR-BI mediates uptake of HDL-derived lipids without significantly affecting degradation of HDL protein in a process called selective uptake (1,(5)(6)(7). The mechanism by which SR-BI mediates selective lipid uptake from HDL is not entirely understood.…”

Section: Scavenger Receptor B Type I (Sr-bi)mentioning

confidence: 99%

A Carboxyl-terminal PDZ-interacting Domain of Scavenger Receptor B, Type I Is Essential for Cell Surface Expression in Liver

Silver

2002

109

107

Scavenger receptor B, type I (SR-BI) was recentlyshown to interact with a PDZ domain-containing protein, PDZK1 (CLAMP/Diphor-1/CAP70/NaPi-Cap1), but the importance of this interaction in vivo in terms of SR-BI function has not been determined. In an effort to elucidate the role of this interaction in vivo, the PDZK1-interacting domain of SR-BI was identified and mutated and expressed liver-specifically in mice. The PDZKI-interacting domain on SR-BI was identified as the last three carboxyl-terminal amino acids, Arg-Lys-Leu. A mutant SR-BI (SR-BIdel509) that lacked only the leucine in the PDZ-interacting domain failed to interact with PDZK1 in vitro, while showing normal selective uptake function in nonpolarized cells. Transgenic mice with liver overexpression of SR-BIdel509 showed marked accumulation of SR-BI mRNA with only a moderate increase in SR-BI protein in liver, with no reduction in plasma cholesterol levels. Measurement of cell surface SR-BI levels and HDL cholesteryl ester-selective uptake in primary hepatocytes from transgenic mice revealed that SR-BIdel509 was not expressed at the plasma membrane correlating with normal levels of selective uptake compared with hepatocytes from nontransgenic littermates. This study indicates that the PDZK1-interacting domain of SR-BI is essential for cell surface expression of SR-BI in liver and suggests that PDZK1 or other PDZ domain proteins may play an important role in regulating SR-BI cell surface expression and hence reverse cholesterol transport. Scavenger receptor B type I (SR-BI)1 is a receptor for high density lipoprotein (HDL) belonging to the CD36 family of transmembrane proteins. SR-BI is highly expressed in the liver and steroidogenic tissues and expressed to lower levels in the intestine and vasculature in adult mice (1-4). SR-BI mediates uptake of HDL-derived lipids without significantly affecting degradation of HDL protein in a process called selective uptake (1, 5-7). The mechanism by which SR-BI mediates selective lipid uptake from HDL is not entirely understood. However, structure/function studies in cell culture using domain swaps with CD36 in addition to point mutations of SR-BI have shown that the extracellular portion of SR-BI contains the HDL binding domain that is essential for selective uptake (8 -11). Additional studies have demonstrated that deletion of the 45-amino acid carboxyl terminus of SR-BI did not reduce selective uptake (12). Taken together, the data indicate that the extracellular domain mediates selective uptake, whereas the carboxyl terminus of the protein is dispensable for SR-BI function. Although it is clear from these studies in cell culture that the carboxyl terminus of SR-BI is not essential for mediating selective uptake, this sequence may contain essential domains for SR-BI activity in vivo. The carboxyl terminus of transmembrane proteins often contain membrane targeting signals, signaling domains, and protein-protein interaction motifs.Recently, a PDZ domain-containing protein called CLAMP (PDZK1/Diphor-1/CAP70/NaPi-Ca...