Proteolytic susceptibility of both isolated and bound light chains from various myosins to myopathic hamster protease

Two heads are required for phosphorylation-dependent regulation of smooth muscle myosin

Recent structural evidence (Rayment, I., Holden, H. M., Whittaker, M., Yohn, C. B., Lorenz, M., Holmes, K. C., and Milligan, R. A. (1993) Science 261, 58-65) suggests that the two heads of skeletal muscle myosin interact when the protein is bound to filamentous actin. Direct chemical cross-linking experiments show that the two heads of smooth muscle myosin interact in the presence of filamentous actin and the absence of ATP (Onishi, H., Maita, T., Matsuda, G., and Fujiwara, K. (1992) Biochemistry 31, 1201-1210). Head-head interactions may be important in the mechanism of phosphorylation-dependent regulation of smooth muscle myosin. To explore the structural elements essential for phosphorylation-dependent regulation, we purified a proteolytic fragment of chicken gizzard myosin containing only one head attached to an intact tail. This molecule contained a partially digested regulatory light chain, which was replaced with exogenously added intact light chain in either the thiophosphorylated or the unphosphorylated state. Control experiments showed that this replacement was nearly quantitative and did not alter the actin-activated ATPase of this myosin. Electron micrographs confirmed that the single-headed preparation contained an intact form of single-headed myosin. The unphosphorylated single-headed myosin hydrolyzed ATP rapidly and moved actin filaments in an in vitro motility assay. Phosphorylation had minimal effects upon these properties. Therefore, we conclude that phosphorylation-dependent regulation in this myosin requires two heads. These findings may have important implications in studies of other regulated motor proteins that contain two motor domains.

LC20 and kinetics of gizzard myosin subfragment-1: digestion with papain vs. S. aureus protease

Previous reports have shown that papain-digested gizzard subfragment-1 (PAP-S1) has a cleaved regulatory light chain (LC20), and Vmax similar to phosphorylated heavy meromyosin (HMM) (Greene et al., Biochemistry 22:530-535, 1983; Sellers et al., J. Biol. Chem. 257:13880-13883, 1982; Umemoto et al., J. Biol. Chem. 264:1431-1436, 1989], while S. aureus protease-digested S-1 (SAP-S1) has intact LC20, but Vmax closer to that of unphosphorylated HMM [Ikebe and Hartshorne, 1985]. To determine whether intact LC20 inhibits ATPase activity for subfragment-1 (S1), we compared the kinetic properties and structures of unphosphorylated PAP-S1 and SAP-S1. SDS-PAGE showed that SAP-S1 had 68 and 24 KDa heavy chain and 20 and 17 KDa light chain components. PAP-S1 (15 minutes digestion at 20 degrees C) also had 68 and 17 KDa bands, but the single 24 KDa band (24HC) was replaced by a group of 22-24 KDa fragments and LC20 was cleaved to a 16 KDa fragment. At 13 mM ionic strength, both PAP-S1 and SAP-S1 had Vmax similar to phosphorylated HMM (1.1-1.5 s-1). SAP-S1 had the same KATPase as phosphorylated HMM (38 microM actin), but KATPase for PAP-S1 was 3-fold stronger (11 microM actin). Subsequent digestion of SAP-S1 with papain did not significantly change Vmax, but as LC20 and 24HC were cleaved, both KATPase and Kbinding strengthened 3- to 5-fold. Thus, intact LC20 did not inhibit, and cleavage of LC20 did not increase Vmax for S1. Rather, papain cleavage of LC20 and 24HC was associated with strengthened actin binding.

Formation of new quasi-crystalline ordered aggregates by gizzard myosin

Turkey gizzard myosin was found to self-assemble into new polymorphic forms as detected by thin-section electron microscopy. In high ionic strength buffers (0.3 mM KCl, pH 6.0), aggregates of sidepolar filaments were produced. Electron microscopy of thin sections revealed individual filaments with a 13.5 nm axial repeat. Under a number of conditions, with varying ionic strength, pH, MgCl2 and ATP, the filaments assembled through the head region with the tail portion projecting out radially from the aggregate. The regions corresponding to heads and tails within the aggregates were established by immunoelectron microscopy using anti-S1 and anti-LMM antibodies coupled to gold. These filaments often interacted to produce bilayer sheets, which, when cut perpendicular to the plane of the sheet, appeared as ladders. A hitherto unreported structure was obtained at 0.2 M KCl (pH 8.0): myosin aggregated to generate a three-dimensional quasi-crystalline lattice with a 270 nm period. In these aggregates, myosin was arranged in an antiparallel fashion, stacked on one another, producing ribbon-like strips stabilized through non-covalent interactions between heads, thereby producing a crystalline lattice. Neither Mg2+ nor ATP were required for this form. Phosphorylation of the regulatory light chains or the cleavage of the heavy chains at a single site in the head region prevented myosin from assembling in the 3-D lattice form. Generally, unphosphorylated myosin produced periodic paracrystals at low ionic strength in the presence of 10 mM MgCl2, but as the ionic strength was increased the regular 3-D lattice became the predominant form. Some paracrystalline forms could be obtained at high ionic strength without magnesium with phosphorylated myosin.