Biosynthesis and cell surface localization of nonglycosylated human histocompatibility antigens

Abstract

The effects of the inhibitor of N-linked glycosylation, tunicamycin, on the synthesis of HLA-A and -B antigens in the human lymphoblastoid cell line JY are described. HLA-A and -B antigens are membrane glycoproteins that consist of a two chain complex, the heavy chain being glycosylated at Asn 86, whereas the light chain, identical to beta 2-microglobulin, is not glycosylated. HLA-A and -B antigens synthesized in the presence of the antibiotic are devoid of carbohydrate. This lack of carbohydrate does not affect the association of the heavy and light chains, nor does it affect the reactivity with human alloantisera, or a mouse monoclonal antibody W6/32, that reacts with all HLA-A and -B specificities examined so far. Nonglycosylated HLA-A and -B antigens are no more susceptible to proteolysis with trypsin, chymotrypsin, or papain than their fully glycosylated counterparts. Thus it may be concluded that the carbohydrate side chains of HLA-A and -B antigens do not contribute significantly to the conformation of HLA-A and -B antigens, at least as measured by these procedures. Pulse-chase experiments, in conjunction with the isolation of cell-surface HLA-A and -B antigens by adsorbing the monoclonal antibody W6/32 to intact cells, indicate that nonglycosylated molecules reach the cell surface at a rate indistinguishable from that of fully glycosylated molecules (although the absolute amount synthesized in the presence of tunicamycin was decreased). Thus glycosylation is also not required for membrane insertion of HLA antigens, nor for their subsequent transport to the cell surface.