MOLECULAR-CLONING AND SEQUENCING OF A 58-KDA MEMBRANE-ASSOCIATED 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 mem brane- 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 w ith a high M(r) glycoprotein complex containing the (proto)oncogene p1 85(neu) and other signal transduction components in a putative microfi lament-associated signal transduction particle. Amino acid sequences w ere obtained from two trypsin peptides of p58. Screening a MAT-C1 cDNA library with a degenerate oligonucleotide derived from the larger pep tide and polymerase chain reaction amplification of cDNA ends permitte d the isolation of overlapping cDNAs encoding the 427-amino acid open reading frame of p58. In vitro transcription and translation using a f ull-length cDNA gave a protein of approximately 55 kDa, which reacts w ith 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 locali zed in a punctate distribution. In addition, the transfected cells exh ibited 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 correspondi ng regions of the Gag proteins for Moloney murine leukemia virus indic ated that about 60% of their amino acid residues were identical. These studies suggest that p58 is the product of an endogenous retroviral g ene whose expression as a cellular protein alters the properties of th e tumor cell to provide a selective advantage for tumor growth in the animal.