title: Actin and tropomyosin variants in smooth muscles. Dependence on tissue type.
abstract: Actin was found to be the major source of myofibrillar protein heterogeneity in smooth muscles. Three isoelectric variants, alpha-smooth muscle (alpha-SM), beta-non-muscle (beta-NM), and gamma-actins (gamma-SM and gamma-NM) were measured in 15 different smooth muscles, alpha-SM and gamma-actin contents displayed an inverse relationship in a given smooth muscle, some of which contained primarily alpha-SM actin while gamma-actins dominated in others. alpha-SM actin and gamma-actin distributions were tissue-specific, independent of species. A greater proportion of alpha-SM actin appears to be associated with tissues having a high degree of tonic activity. beta-Nonmuscle actin was a significant, and relatively constant, component of all smooth muscle tissues. The high NM-actin content of these tissues may reflect the importance of proliferative, synthetic, or secretory activities in smooth muscle, because the alpha-SM actin disappeared in tissue culture with a time course paralleling the modulation of phenotype from a contractile to a proliferative cell. Two tropomyosin subunits were present in approximately equal amounts in all smooth muscle tissues studied. One tropomyosin subunit exhibited identical mobility on two-dimensional gel electrophoresis, while the other was characterized by some species-specific variation which was unrelated to actin variant distribution. No variants of the 20,000-dalton regulatory light chain of myosin were observed. These results suggest that SM-specific actin variants are associated with functional diversity among smooth muscles.
mesh_terms: D000199:Actins / Q000302:isolation & purification*; D000818:Animals; D002415:Cats; D004285:Dogs; D014644:Genetic Variation; D009130:Muscle, Smooth / Q000032:analysis*; D009131:Muscle, Smooth, Vascular / Q000032:analysis*; D009893:Opossums; D009928:Organ Specificity; D011817:Rabbits; D013552:Swine; D014335:Tropomyosin / Q000302:isolation & purification*
chemical_list: D000199:Actins; D014335:Tropomyosin