ABSTRACT
Myorod (MR), a new thick filament protein of molluscan smooth muscles, is an alternatively spliced product of the myosin (Mn) heavy chain gene. We studied digestion of MR and Mn from the posterior adductor of Crenomytilus grayanus and the outer portion of adductor of Mizuchopecten (Patinopecten) yessoensis by papain and constructed the proteolytic substructure of MR, that is an analogue to Mn substructure. There are a head domain (analogue of Mn S1) and a rod domain (analogue of Mn rod); the junction between them is split at low ionic strength. The rod, in turn, consists of a neck domain (analogue of Mn S2) and a tail domain (identical to light meromyosin); the junction between them is split at high ionic strength. The localization and possible function of MR are discussed.
Subject(s)
Mollusca/chemistry , Muscle, Smooth/chemistry , Myosin Heavy Chains/chemistry , Alternative Splicing , Animals , Bivalvia/chemistry , Electrophoresis, Polyacrylamide Gel , Molecular Weight , Myosin Heavy Chains/metabolism , Myosins/chemistry , Myosins/metabolism , Osmolar Concentration , Papain/metabolism , Peptide Fragments/chemistry , Peptide Mapping , Protein Structure, TertiaryABSTRACT
Myorod, a new protein of molluscan smooth muscles, is localized on the surface of paramyosin core of thick filaments together with myosin [Shelud'ko et al. (1999) Comp Biochem Physiol, 122, 277]. This protein is an alternatively spliced product of the myosin heavy chain gene. It contains the C-terminal rod part of myosin and a unique N-terminal domain [Yamada et al. (2000) J Mol Biol, 295, 169]. In the present study, the methods of myorod and myorod-free myosin preparation are developed and some properties of myorod are compared with those of myosin and myosin rod. We found that, in spite of the identity of filament-forming domains, the properties of polymeric myorod are clearly distinct from those of myosin and myosin rod. Myorod is much more soluble at intermediate ionic strength. The critical monomer concentration for polymerization of myorod is many times higher. The size of polymer particles of myorod is considerably smaller than that of myosin and myosin rod. The pure polymeric myorod forms a low turbid and unexpectedly high viscous suspension. The low-shear intrinsic viscosity of myorod is an order of magnitude higher than that of myosin or myosin rod and is close to that of F-actin. A trace admixture of myosin in myorod preparations or a small addition of myosin (0.2-1.0%) to myorod drastically alters the myorod polymerization. The suspensions of polymeric myorod nucleated by myosin have a high turbidity and low viscosity and consist of large particles. As judged from the changes in particle size distribution during polymerization, these particles are formed by successive dimerization steps. Electron micrographs show that the particles are typically spindle-shaped filaments in contrast to polymers of pure myorod which forms a network-like structure consisting of small particles. Possible participation of myorod in the catch-contraction of molluscan smooth muscles is discussed.
