Molecular Properties and Functions In Vitro of Chicken Smooth-Muscle α-Actinin in Comparison with Those of Striated-Muscle α-Actinins1

Abstract

α-Actinin purified from chicken gizzard smooth muscle was characterized in comparison with α-actinins from chicken striated muscles, or fast-skeletal muscle, slow-skeletal muscle, and cardiac muscle. The gizzard α-actinin molecule consisted of two apparently identical subunits with a molecular weight of 100,000 on SDS-polyacrylamide gel electrophoresis, as do striated-muscle α-actinins. Its isoelectric points in the presence of urea were similar to the striated-muscle counterparts. Despite these similarities, distinctive amino acid sequences between smooth-muscle α-actinin and striated-muscle α-actinins were revealed by peptide mapping using limited proteolysis in SDS. Gizzard α-actinin was immunologically distinguished from striated-muscle α-actinins. Gizzard α-actinin formed bundles of gizzard F-actin as well as of skeletal-muscle F-actin, but could not form any cross-bridges between adjacent actin filaments under conditions where skeletal-muscle α-actinin could. Temperature-dependent competition between gizzard α-actinin and tropomyosin on binding to gizzard thin filaments was demonstrated by electron microscopic observations. Gizzard α-actinin promoted Mg²⁺-ATPase activity of reconstituted skeletal actomyosin, gizzard acto-skeletal myosin, and gizzard actomyosin. This promoting effect was depressed by the addition of gizzard tropomyosin. These findings imply that, despite structural differences between gizzard and striated-muscle α-actinin molecules, they function similarly in vitro, and that gizzard α-actinin can interact not only with smooth-muscle actin (γ- and β-actin) but also with skeletal-muscle actin (α-actin).