Identification and characterization of the tumor‐specific P1A gene product

Summary— In murine mastocytoma P815, gene P1A directs the expression of antigens P815A and B which are the target of a T cell‐mediated rejection response in syngeneic animals. This gene is expressed at a high level in various tumors, but is silent in normal tissues except testis and placenta; its activation is thus possibly related to malignant transformation. An anti‐synthetic peptide rabbit antiserum reacted by immunoblotting with a cellular protein migrating near 40 kDa on SDS‐PAGE. The immunoreactive protein was detected only in lysates from cells which express antigen P815A: P1.HTR mastocytoma cells and, after transfection with cosmids carrying the P1A gene, the antigen‐loss variant PO.HTR cells and DAP‐3 H‐2L fibroblasts. The identity of this protein as the P1A gene product was confirmed by cell‐free transcription‐translation of the P1A cDNA, the product of which also migrated near 40 kDa in SDS‐PAGE and was captured by protein A‐Sepharose in the presence of the antiserum. Subcellular fractionation by differential and isopycnic centrifugation indicated that the P1A protein is associated with cytoplasmic membranes demonstrating a broad distribution with respect to size and density. Immunofluorescence microscopy also revealed a cytoplasmic signal, particularly intense in small vesicles, which coincides with that produced by an anti‐mouse type I collagen guinea pig antiserum except near the cell periphery where the P1A signal is weaker. We conclude that the P1A protein is bound to membranes of the secretory pathway, at a concentration which goes increasing from the endoplasmic reticulum to secretion vesicles. The N‐terminal portion of the protein was readily removed by proteolytic enzymes in the absence of detergent, suggesting a localization at the cytoplasmic surface. The P1A protein is renewed with a half‐life of 50 mm and is readily phosphorylated upon metabolic labeling of mastocytoma cells with [32P]‐orthophosphate. When immunoisolated from cells lysed with Nonidet P‐40, the P1A protein is accompanied by a 62‐kDa protein which also exists in a phosphorylated form. Thus, it is probably associated with another phosphoprotein, either as a stable functional unit, or as a dissociable complex.