Molecular cloning and sequencing of a 58-kDa membrane- and microfilament-associated protein from ascites tumor cell microvilli with sequence similarities to retroviral Gag proteins.

The MAT-C1 subline of the 13762 rat mammary adenocarcinoma has highly stable, branched microvilli and immobile cell surface receptors. A membrane- and microfilament-associated 58-kDa protein (p58) in the MAT-C1 microvilli has been implicated in the stabilization of the microvilli and microfilament-membrane interactions. This protein is associated with a high M(r) glycoprotein complex containing the (proto)oncogene p185neu and other signal transduction components in a putative microfilament-associated signal transduction particle. Amino acid sequences were obtained from two trypsin peptides of p58. Screening a MAT-C1 cDNA library with a degenerate oligonucleotide derived from the larger peptide and polymerase chain reaction amplification of cDNA ends permitted the isolation of overlapping cDNAs encoding the 427-amino acid open reading frame of p58. In vitro transcription and translation using a full-length cDNA gave a protein of approximately 55 kDa, which reacts with anti-p58 antiserum and reconstitutes into a complex with actin and glycoproteins from the membrane-microfilament interaction site. When COS-7 cells were transfected with the full-length cDNA, p58 was localized in a punctate distribution. In addition, the transfected cells exhibited fewer microfilament cables than untransfected neighboring cells. The amino acid sequence showed a surprising similarity to mammalian retroviral Gag proteins and included regions corresponding to p15, p12 and the N-terminal 80% of p30. Comparisons of p58 and the corresponding regions of the Gag proteins for Moloney murine leukemia virus indicated that about 60% of their amino acid residues were identical. These studies suggest that p58 is the product of an endogenous retroviral gene whose expression as a cellular protein alters the properties of the tumor cell to provide a selective advantage for tumor growth in the animal.