A monoclonal antibody (CG1) which recognizes tropomyosin isoforms 1 and 3 of chicken embryo fibroblasts was used to detect what is a motility-dependent change in the availability of the antigenic determinant in tropomyosin molecules along microfilaments. Immunofluorescence microscopy with this antibody revealed a heterogenous staining pattern among chicken embryo fibroblasts cells such that a population (17%) of cells showed only background staining. Stress fibers in about half the population of the cells stained weakly with this antibody, while the stress fibers in another population of cells (35%) showed very strong staining. After glycerination or cytochalasin B treatment, all of the cells became positive in reaction to CG1 antibody, suggesting that the antigenic determinant was present in every cell. On the other hand, all of the cells after brief nonionic detergent treatment became negative to CG1 antibody. The CG1 staining pattern was not significantly changed in cells at different stages after release from colcemid blockage, nor was a brief treatment of cells with buffer containing 2 M urea, mild trypsin, chymotrypsin, or V.8 protease effective in changing the reactivity. However, most of the cells with a morphology typical of movement, and all of the contracted, glycerinated cells were strongly positive to CG1 antibody. These results suggest that the unmasking of the CG1 determinant may be motility-dependent. Immunoblot analysis showed that forced modification on the cysteine residue of tropomyosin molecules, caused either by performic acid oxidation or by disulfide cross-linking with the chemical 5,5'-dithiobis (2-nitrobenzoate), results in drastic changes in the reactivity of the different isoforms to CG1 antibody. These results indicate that the cysteine residue is involved in the CG1 determinant. The motility-dependent unmasking of this determinant may suggest an important role for nonmuscle tropomyosin in regulating cell motility.

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