A mutation in actin associated with neoplastic transformation

A new protein was recognized in a chemically transformed human fibroblast cell line when its proteins labeled with [35S]methionine were compared with those from normal human fibroblasts by two-dimensional gel electrophoresis. The new protein was found in the Triton-insoluble cytoskeletal fraction as well as in the Triton-soluble fraction, and it migrated very closely to beta- and gamma-actins on the gels. This new protein was identified as a variant form of actin by its reaction with antiactin antibody and its tryptic peptide pattern, which was identical to actin. mRNA coding for the variant actin was detected only in this particular transformed line. The size and cross- hybridizability with Dictyostelium actin cDNA of mRNA coding for the variant actin and complete amino acid sequence of the variant actin indicate that the new variant actin is the product of a mutated beta-actin gene. Only a single amino acid (glycine) at position 244 was replaced by aspartic acid. This substitution corresponds to a GC----AT transition, a point mutation. On the other hand, a highly malignant cell variant was isolated from the transformed line. The mutated beta-actin was further altered in this highly malignant subclone: it showed a more negative charge, rapid synthetic rate, and a short half-life in the cells. Incorporation into the cytoskeleton was significantly reduced in the mutated beta-actin. A hypothesis on the relationship between a mutation in the actin gene and oncogenic transformation was proposed.