An 80 to 85 kilodalton human phosphoglycoprotein associated with cell activation

This report describes the identification and biochemical characterization of a new proliferation- and activation-associated membrane Ag. The M21C5 Ag (Mr 80 to 85 kDa) initially was immunoprecipitated from 125I-cell surface-labeled HT29 human tissue culture colon cancer cells by using a monoclonal antibody (M21C5) prepared from HT-29 immunized BALB/c mice. The M21C5 Ag is a glycoprotein as shown by metabolic labeling with 3H-leucine and 2-[3H]-mannose. It has a broad distribution on most proliferating tissue culture cell lines tested, but is absent from several normal human tissues that were examined. Although not detected on unstimulated PBL, the expression of the M21C5 Ag could be induced by stimulation of PBL with the T cell mitogens PHA or Con A. Two-color fluorescence analysis showed that M21C5 is expressed on both CD4 and CD8 activated T cells. After mitogen stimulation, the expression of the M21C5 Ag was delayed relative to the expression of IL-2 and transferrin receptors. M21C5 glycoprotein was shown to be an integral membrane protein that is phosphorylated primarily on serine residues. Based on its biochemical and tissue distribution properties, M21C5 phosphoglycoprotein appears distinct from other known proliferation and activation-associated molecules.