Identification of new actin-associated polypeptides that are modified by viral transformation and changes in cell shape.

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Abstract
By using a monoclonal antibody we have identified a new polypeptide doublet (C4h and C41) of Mr .apprx. 21 kD and pI 8 and 7, respectively, heat is associated with and (at the immunofluorscence level) uniformly distributed on actin filament bundles in rat, mouse, and other vertebrate species. C4 is absent in neurons, erythrocytes, and skeletal muscle but the epitope is evolutionary conserved as it is present in invertebrates such as molluscs and crustaceans. C4h is not found in cells such as lymphocytes and oncogenically transformed mesenchymal cells where actin stress fiber bundles are reduced in number or absent. C41, on the other hand, is always present. C4h expression can also be blocked by switching normal nontransformed mesenchymal cells from adherent to suspension culture. Reexpression of C4h occurs 24 h after these cells are returned to normal adherent culture conditions, but can be blocked by either actinomycin D or cycloheximide, suggesting that the expression of this epitope is regulated at the transcriptional level.