Identification of the N-linked glycosylation sites on the high density lipoprotein (HDL) receptor SR-BI and assessment of their effects on HDL binding and selective lipid uptake.
The murine class B, type I scavenger receptor mSR-BI, a high density lipoprotein (HDL) receptor that mediates selective uptake of HDL lipids, contains 11 potential N-linked glycosylation sites and unknown numbers of both endoglycosidase H-sensitive and -resistant oligosaccharides. We have examined the consequences of mutating each of these sites (Asn --> Gln or Thr --> Ala) on post-translational processing of mSR-BI, cell surface expression, and HDL binding and lipid transport activities. All 11 sites were glycosylated; however, disruption of only two (Asn-108 and Asn-173) substantially altered expression and function. There was very little detectable post-translational processing of these two mutants to endoglycosidase H resistance and very low cell surface expression, suggesting that oligosaccharide modification at these sites apparently plays an important role in endoplasmic reticulum folding and/or intracellular transport. Strikingly, although the low levels of the 108 and 173 mutants that were expressed on the cell surface exhibited a marked reduction in their ability to transfer lipids from HDL to cells, they nevertheless bound nearly normal amounts of HDL. Indeed, the affinity of (125)I-HDL binding to the 173 mutant was similar to that of the wild-type receptor. Thus, N-linked glycosylation can influence both the intracellular transport and lipid-transporter activity of SR-BI. The ability to uncouple the HDL binding and lipid transport activities of mSR-BI by in vitro mutagenesis should provide a powerful tool for further analysis of the mechanism of SR-BI-mediated selective lipid uptake.
Viñals
Marisa
M
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Xu
Shangzhe
S
Vasile
Eliza
E
Krieger
Monty
M
D000818:Animals; D018955:CD36 Antigens / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D006031:Glycosylation; D050356:Lipid Metabolism; D008075:Lipoproteins, HDL / Q000378:metabolism*; D008565:Membrane Proteins; D051379:Mice; D016296:Mutagenesis; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D051116:Receptors, Scavenger; D051122:Scavenger Receptors, Class B; D015398:Signal Transduction
D018955:CD36 Antigens; D008075:Lipoproteins, HDL; D008565:Membrane Proteins; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; C454739:Scarb1 protein, mouse; D051122:Scavenger Receptors, Class B