IDENTIFICATION OF A 34,000-DALTON MITOGENIC PROTEIN ASSOCIATED WITH PLASMA-MEMBRANES FROM HUMAN-A431 EPIDERMOID CARCINOMA-CELLS

Evidence is presented for a discrete 34,000-Da [dalton] polypeptide with mitogenic activity, associated with plasma membranes from human A431 carcinoma cells. Plasma membranes from A431 cells are highly mitogenic for quiescent fibroblasts. A significant fraction of the membrane-associated activity can be released by treatment with a high concentration of salt and is relatively acid stable. Incubation of 125I-labeled salt-extracted proteins with target fibroblasts results in preferential binding of a 34,000-Da protein, i.e., > 90% of the cell-bound radioactivity is associated with the 34,000-Da polypeptide. Studies correlating the mitogenic activity with the cell-binding 34,000-Da protein indicate that this protein is the acid-stable, peripherally attached, mitogenically active component of A431 membranes. This protein may be mitogenically active in the nanomolar concentration range. Some properties of this protein are described.