title: Molecular mechanics of two smooth muscle heavy meromyosin constructs that differ by an insert in the motor domain.
abstract: Phasic and tonic smooth muscles express two myosin heavy chain isoforms that differ by only a seven amino acid insert in a flexible surface loop located near the nucleotide-binding site. The inserted isoform found predominantly in phasic muscle has two times the actin-activated ATPase activity and in vitro actin filament velocity as the non-insert isoform found mainly in tonic muscle (Kelley, C.A., Takahashi, M., Yu, J.H. & Adelstein, R.S. 1993. J Biol Chem 268, 12848, Rovner, A.S., Freyzon, Y. & Trybus, K.M. 1997. J Musc Res Cell Motil 18, 103). We used a laser trap to characterize the molecular mechanics of the inserted isoform [(+)insert] and of a mutant lacking the insert [(-)insert], which is analogous to the isoform found in tonic muscles. The constructs were expressed in the baculovirus/insect cell system. Unitary displacements (Duni) were similar for both the constructs (approximately 10 nm) but the attachment time (ton) for the (-)insert was two times that of the (+)insert. These data suggest that the insert in the nucleotide-binding loop does not affect the inherent mechanics of the myosin molecule but rather the kinetics of the cross-bridge cycle.
authors.affiliation: Department of Molecular Physiology and Biophysics, University of Vermont, Burlington 05405, USA.
authors.forename: A M
authors.initials: AM
authors.lastname: Lauzon
authors.forename: K M
authors.initials: KM
authors.lastname: Trybus
authors.forename: D M
authors.initials: DM
authors.lastname: Warshaw
mesh_terms: D000199:Actins / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002021:Buffers; D000070559:Gizzard, Non-avian / Q000378:metabolism; D066298:In Vitro Techniques; D007536:Isomerism; D007700:Kinetics; D009119:Muscle Contraction / Q000502:physiology; D009130:Muscle, Smooth / Q000378:metabolism* / Q000648:ultrastructure; D015879:Myosin Subfragments / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D009218:Myosins / Q000378:metabolism
chemical_list: D000199:Actins; D002021:Buffers; D015879:Myosin Subfragments; D000255:Adenosine Triphosphate; D009218:Myosins