CD4, a 55KD monomeric membrance-bound glycoprotein expressed on the surface of T helper cells, mediates helper T cell activation and is also the receptor for the human immunodeficiency virus (HIV) . The extracellular segment forms four VJ-like regions (VI-V4). Two potential N-linked glycosylation sites are present: one at the carboxyl end of V3 and the other near the amino end of V4. Previous work has suggested that glycosylation of CD4 is necessary for cell surface expression of the receptor. To clarify the effect of glycosylation on CD4 expression we have created a series of mutant CD4 cDNA's by site-specific mutagenesis which delete one, the other or both potential glycosylation sites. By in vitro transcription and translation we have confirmed that both potential glycosylation sites of CD4 are utilized. We have demonstrated by fluorescence-activated cell sorting (FACS) analysis that oligosaccharide deficient CD4 proteins are expressed poorly or not at all at the cell surface and that glycosylation at the carboxyl terminal site appears to be critical for protein expression. Finally, we have shown that cells transfected with CD4 cDNA with the carboxyl glycosylation site eliminated produce no immunoprecipitable protein suggesting that such proteins may be structurally abnormal and rapidly degraded.
