A variant of human nonmuscle tropomyosin found in fibroblasts by using two-dimensional electrophoresis

In an analysis of 12 human fibroblast cell lines by two-dimensional electrophoresis, one cell line (1493) was found to contain a major protein variant (Cytosk: 12; for the convention used in spot numbering, see Anderson, N. L. (1981) Proc. Natl. Acad. Sci. U. S. A. 78, 2407-2411) not present in any of the other 11 cell lines. Biochemical characterization of the variant protein included determination of its subcellular location, partial amino acid composition, behavior on sodium dodecyl sulfate (SDS) versus SDS/urea gels, and partial proteolytic digestion patterns. All of these methods showed that Cytosk:12 is related to Cytosk:11. Both proteins are located in the cytoskeleton, contain little cysteine or proline and no detectable tryptophan, shift together to a higher apparent molecular weight when electrophoresed in the presence of SDS and 8 M urea versus SDS alone, and have identical products after partial proteolysis according to Cleveland et al. (Cleveland, D. W., Fischer, S. G., Kirschner, M. W., and Laemmli, U. K. (1977) J. Biol. Chem. 252, 1102-1106). Preparation of nonmuscle tropomyosin resulted in the partial purification of both Cytosk:11 and :12. The data suggest that Cytosk:11 is fibroblast nonmuscle tropomyosin and that Cytosk:12 in cell line 1493 is a charge variant of that protein.