Coexistence of three major isoactins in a single sarcoma 180 cell.

Actin is transformed sarcoma 180 cells is composed of the nonmuscle beta and gamma species and of a third, more acidic stable variant termed zeta. Two-dimensional peptide analysis shows that zeta is similar to beta actin, differing in the mobility of only one tryptic peptide. Several lines of evidence indicate that zeta is not a modified beta-actin species. This third actin species comprises 20% of the total labeled actin, has the same molecular weight as the beta and gamma actins and has a different mobility in isoelectric focusing gels from that of the known alpha actins from skeletal, cardiac and vascular smooth muscle. Like beta and gamma actin, zeta can be extracted with the actin depolymerizing factor from slime mold. Two-dimensional gel electrophoresis (isoelectric focusing) of the 35S-methionine-labeled polypeptides synthesized by a single sarcoma 180 cell showed that all three major actin species coexist within the same cell. This analysis also showed for the first time the coexistence and alpha and beta tubulin, vimentin, alpha actinin and three other polypeptides present in intermediate-filament-enriched cytoplast cytoskeletons (spots 12, 24 and 31). Determination of the ratio of gamma plus beta to zeta actin in different cytoskeletal preparations of intact and enucleated sarcoma 180 cells indicated that this actin species is not localized specifically to any of the major actin-containing structures preserved in the cytoskeletons.