The Human Scavenger Receptor CD36: glycosylation status and its role in trafficking and function

Human CD36 is a class B scavenger receptor expressed in a variety of cell types such as macrophage and adipocytes. This plasma membrane glycoprotein has a wide range of ligands including oxidized low density lipoprotein and long chain fatty acids which involves the receptor in diseases such as atherosclerosis and insulin resistance. CD36 is heavily modified post-translationally by N-linked glycosylation, and 10 putative glycosylation sites situated in the large extracellular loop of the protein have been identified; however, their utilization and role in the folding and function of the protein have not been characterized. Using mass spectrometry on purified and peptide N-glycosidase F-deglycosylated CD36 and also by comparing the electrophoretic mobility of different glycosylation site mutants, we have determined that 9 of the 10 sites can be modified by glycosylation. Flow cytometric analysis of the different glycosylation mutants expressed in mammalian cells established that glycosylation is necessary for trafficking to the plasma membrane. Minimally glycosylated mutants that supported trafficking were identified and indicated the importance of carboxyl-terminal sites Asn-247, Asn-321, and Asn-417. However, unlike SRBI, no individual site was found to be essential for proper trafficking of CD36. Surprisingly, these minimally glycosylated mutants appear to be predominantly core-glycosylated, indicating that mature glycosylation is not necessary for surface expression in mammalian cells. The data also show that neither the nature nor the pattern of glycosylation is relevant to binding of modified low density lipoprotein.