Co-existence of vinculin and a vinculin-like protein of higher molecular weight in smooth muscle.

Recently, a protein component of adhesion plaques with a molecular weight of 130,000 (named vinculin) has been purified from smooth muscle and non-muscle cells. As detected by immunological methods, the only vinculin-related polypeptides in fibroblasts are proteins of Mr = 130,000. However, we show here that smooth muscle contains, in addition to vinculin, an apparently distinct protein with a Mr = 152,000 that shares both structural and immunological features with vinculin. Amino acid analysis, peptide mapping, and antibody cross-reaction studies were used to elucidate these similarities. Mr = 152,000 protein seems to be restricted to muscle (mainly or exclusively to smooth muscle). The possibility that vinculin is derived from proteolytic processing of the Mr = 152,000 protein or that the proteins are related by some other type of post-translational modification appears unlikely (although this cannot be completely ruled out) since both proteins are made in a rabbit reticulocyte cell-free translation system when mRNA derived from smooth muscle is used as the template. Both proteins are capable of used as the template. Both proteins are capable of lowering the viscosity of F-actin solutions, although the activity of the Mr = 152,000 protein is stimulated by Ca2+ while the activity of smooth muscle vinculin is not.