title: 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.
abstract: 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 → 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 (∼ 2-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).
authors: Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors: Leonid
authors: L
authors: Gaidukov
authors: Andrew R
authors: AR
authors: Nager
authors: Shangzhe
authors: S
authors: Xu
authors: Marsha
authors: M
authors: Penman
authors: Monty
authors: M
authors: Krieger
mesh_terms: D020816:Amino Acid Motifs; D019943:Amino Acid Substitution; D000818:Animals; D001692:Biological Transport / Q000502:physiology; D016466:CHO Cells; D019556:COS Cells; D002522:Chlorocebus aethiops; D006224:Cricetinae; D003412:Cricetulus; D006801:Humans; D008075:Lipoproteins, HDL / Q000235:genetics / Q000378:metabolism*; D051907:Lysosomal Membrane Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D020125:Mutation, Missense; D055503:Protein Multimerization / Q000502:physiology*; D017434:Protein Structure, Tertiary; D051116:Receptors, Scavenger / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D051122:Scavenger Receptors, Class B / Q000235:genetics / Q000378:metabolism*
chemical_list: D008075:Lipoproteins, HDL; D051907:Lysosomal Membrane Proteins; D051116:Receptors, Scavenger; D011993:Recombinant Fusion Proteins; C495124:SCARB1 protein, human; C074299:SCARB2 protein, human; C454739:Scarb1 protein, mouse; D051122:Scavenger Receptors, Class B