Department of Medicine, Medical College of Wisconsin, Zablocki Veterans Administration Hospital, Milwaukee 53295, USA.
The major glycoprotein in adipocytes was purified from rat adipocyte membranes by affinity chromatography with wheat germ agglutinin-agarose followed by DEAE-Sepharose ion exchange chromatography. The protein had an apparent molecular weight of 200-kDa when analysed by SDS-PAGE under non-reducing conditions. When electroeluted from the gel, boiled in the presence of beta-mercaptoethanol, and re-analysed by gel electrophoresis, it was found to be composed of 100- and 160-kDa subunits. The N-terminal sequences were determined through 20 amino acids for each subunit, were found to be identical, and were homologous with no previously described protein sequences. The protein was not extractable from the membrane by high salt concentrations, indicating it was an integral membrane protein. Membrane fractionation by differential ultracentrifugation showed it was present predominantly in the plasma membrane fraction. The protein was susceptible to cell surface radiolabelling, further suggesting it was a plasma membrane protein. In summary, the major membrane glycoprotein in adipocytes is a novel 200-kDa heterodimer whose disulfide-linked subunits possess identical N-terminal sequences.