An insert in the motor domain determines the functional properties of expressed smooth muscle myosin isoforms.

Smooth muscle myosin isoforms of the heavy chain and the essential light chain have been hypothesized to contribute to the different shortening velocities of phasic and tonic smooth muscles, and to their different affinities for MgADP. We used the baculovirus/insect cell system to express homogeneous heavy meromyosin molecules differing only in seven amino acid insert (QGPSFSY) in the motor domain near the active site, or in the type of essential light chain isoform. Myosin from tonic rabbit uterine smooth muscle lacks the heavy chain insert, while myosin from phasic chicken gizzard contains it. The properties of a mutant uterine heavy meromyosin with added insert, and a mutant gizzard heavy meromyosin with the insert deleted, were compared with their wild type progenitors. Phosphorylated heavy meromyosins with the insert have a twofold higher enzymatic activity and in vitro motility han heavy meromyosins without the insert. These functional properties were not altered by the essential light chain isoforms. The altered motility caused by the insert implies that it modulates the rate of ADP release, the molecular step believed to limit shortening velocity. The insert may thus account in part for both the lower sensitivity to MgADP and the higher shortening velocity of phasic compared to tonic smooth muscles.