Abstract
The biosynthesis of the heavy chains of two membrane glycoproteins, identified as immunoglobulin M and histocompatibility antigens, has been studied in [ 35S]methionine pulse-chase experiments by one and two-dimensional gel electrophoresis. Terminal sugar addition results in marked shifts in gel mobility that are mainly due to sialic acid addition, since they are sensitive to neuraminidase. These shifts are prevented when the ionophore monensin is present during the chase incubation. We conclude that both membrane IgM † † Abbreviations used: IgM, immunoglobulin M; H2, major mouse histocompatibility antigen. and H2 heavy chains normally pass through the Golgi subsite defined by monensin and acquire terminal sialic acid distal to this site. Analysis of surface-iodinated control and monensin-treated cells indicates that, in the presence of monensin, newly synthesized, incompletely glycosylated IgM and H2 are not transported to the cell surface. Thus these membrane proteins appear to follow the same intracellular pathway as secretory proteins.
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