Abstract
The rat and human forms of the T-cell surface glycoprotein CD4 share a common glycosylation site at the Asn270/271 position but differ with respect to the locations of the second glycosylation sites at Asn159 (rat) and Asn300 (human). The glycosylation of soluble recombinant forms of human and rat CD4 (sCD4) expressed in Chinese hamster ovary cells has been characterized. The most obvious differences between the rat and human sCD4 oligosaccharides were the greater abundance of oligomannose and hybrid oligosaccharides on rat sCD4 and the presence of oligosaccharides carrying a terminal alpha-galactose residue on human sCD4. This is the first report of the occurrence of alpha-galactose residues on a glycoprotein expressed in Chinese hamster ovary cells. Comparison of mutant rat sCD4 molecules with single glycosylation sites and glycopeptides indicated that site-specific and independent processing occurred at each glycosylation site. The glycosylation at the conserved site at Asn270 of rat sCD4 was identical to that seen for the equivalent site in human sCD4, and the oligomannose and hybrid structures were restricted to the nonconserved site at Asn159 in rat sCD4. However, there was more oligosaccharide processing at this site in a truncated form of rat sCD4 consisting of the two NH2-terminal domains. These results indicate that not only the local three-dimensional structure but also domain interactions can influence the processing at individual glycosylation sites.
Highlights
The rat and human formsof the T-cell surface gly- encompassed by the residues 33-62 of the NH,terminal docoprotein CD4 share a common glycosylation site at main mediates virus attachment [3]
The fourthdomain belongs gle glycosylationsites andglycopeptides indicatedthat to theC2 set but itssequence is so divergent that relatedness site-specific and independent processing occurred a t to other IgSF domains is fafrom being self-evident [6]
Treatment with A . ureafaciens siali- rides-The elution profiles of the desialylated oligosacchadase removed the charge from the acidic oligosaccharides, as rides of HsCD4 preparedfrom two separate clones are shown judged by high voltage electrophoresis, for all samples ana- in Fig. 6, a and b
Summary
The CD4 cell surface antigen is of interest as a marker of T lymphocytes that recognize foreign antigens in the context of major histocompatibility Class I1 antigen [1].CD4 is the receptor for the human immunodeficiency virus envelope glycoprotein gp120 [2]. It was concluded that the glycosylation of the four-domain form of RsCD4 was not essentialfor its efficient foldingand secretion but that in the absence of domains 3 and 4 glycosylation played a more critical role [11].these experiments do not precludea functional role forthe oligosaccharides when CD4 reaches the cell surface. The productionof these soluble forms has allowed a detailed analysis of the glycosylation of CD4 in the contextof the well characterized glycosylation potential of the CHO cell line. Th e abbreviations used are: IgSF, immunoglobulin superfamily; CHO, Chinese hamster ovary; gu, glucose units.
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