Coupling of ATPase activity and motility in smooth muscle myosin is mediated by the regulatory light chain.
Smooth muscle myosin acts as a molecular motor only if the regulatory light chain (RLC) is phosphorylated. This subunit can be removed from myosin by a novel method involving the use of trifluoperazine. The motility of RLC-deficient myosin is very slow, but native properties are restored when RLC is rebound. Truncating 6 residues from the COOH terminus of the RLC had no effect on phosphorylated myosin's motor properties, while removal of the last 12 residues reduced velocity by approximately 30%. Very slow movement was observed once 26 residues were deleted, or with myosin containing only the COOH-terminal RLC domain. These two mutants thus mimicked the behavior of RLC-deficient myosin, with the important difference that the mutant myosins were monodisperse when assayed by sedimentation velocity and electron microscopy. The decreased motility therefore cannot be caused by aggregation. A common feature of RLC-deficient myosin and the mutant myosins that moved actin slowly was an increased myosin ATPase compared with dephosphorylated myosin, and a lower actin-activated ATPase than obtained with phosphorylated myosin. These results suggest that the COOH-terminal portion of an intact RLC is involved in interactions that regulate myosin's "on-off" switch, both in terms of completely inhibiting and completely activating the molecule.
Trybus
K M
KM
Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254.
Waller
G S
GS
Chatman
T A
TA
D000199:Actins / Q000502:physiology; D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D008969:Molecular Sequence Data; D009130:Muscle, Smooth / Q000737:chemistry*; D009154:Mutation; D009218:Myosins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D010766:Phosphorylation
D000199:Actins; D000255:Adenosine Triphosphate; D009218:Myosins