Alkali Light Chain Isoforms of Skeletal Muscle Myosin Subfragment-1 and Actin Bundle Formation

Myosin subfragment-1 (S-1) has been shown to induce single actin filaments into bundles (Ando & Scales (1985) J. Biol. Chem. 260, 2321-2327 ; Ando (1987) J. Mol. Biol. 195, 351-358). We examined the influence of the alkali light chains of myosin on the actin bundling. Only S-1 isoenzyme possessing alkali 1 light chain (S-1 (Al)) was found to promote single actin filaments into bundles in presence of 10 to 80 mM NaCI. The other isoenzyme, S-1 (A2), possessing alkali 2 light chain, could not promote it under the same conditions. With alkali light chains exchange experiment, the functional difference was revealed to be responsible for a particular alkali light chain. Electron microscopic observation showed that the distance between adjacent actin filaments in the bundles was about 180A and actin bundles possessed transverse stripes at about 350 to 360A intervals in the long axis of a bundle. The bundle formation was predominant in excess of S-1 to actin, and was disassembled by ADP. addition. All these remarks of actin bundles were consistent with hyper-opalescence type actin bundles reported by Ando ((1987) J. Mol. Biol. 195, 351-358). Cross-linking studies of acto-S-1 indicated that actin bundles seemed to be assembled by binding of adjacent S-1 decorating actin filaments each other. The role of alkali light chains on actin bundle formation is discussed