Vectorial activation of smooth muscle myosin filaments and its modulation by telokin

Abstract

Smooth muscle myosin copurifies with myosin light chain kinase (MLCK) and calmodulin (CaM) as well as with variable amounts of myosin phosphatase. Therefore, myosin filaments formed in vitro also contain relatively high levels of these enzymes. Thus these filaments may be considered to be native-like because they are similar to those expected to exist in vivo. These endogenous enzymes are present at high concentrations relative to myosin, sufficient for rapid phosphorylation and dephosphorylation of the filaments at rates comparable to those observed for contraction and relaxation in intact muscle strips. The phosphorylation by MLCK/CaM complex appears to exhibit some directionality and is not governed by a random diffusional process. For the mixtures of myosin filaments with and without the endogenous MLCK/CaM complex, the complex preferentially phosphorylates its own parent filament at a higher rate than the neighboring filaments. This selective or vectorial-like activation is lost or absent when myosin filaments are dissolved at high ionic strength. Similar vectorial-like activation is exhibited by the reconstituted filament suspensions, but the soluble systems composed of isolated regulatory light chain or soluble myosin head subfragments exhibit normal diffusional kinetic behavior. At physiological concentrations, kinase related protein (telokin) effectively modulates the activation process by reducing the phosphorylation rate of the filaments without affecting the overall phosphorylation level. This results from telokin-induced liberation of the active MLCK/CaM complex from the filaments, so that the latter can also activate the neighboring filaments via a slower diffusional process. When this complex is bound at insufficient levels, this actually results in acceleration of the initial phosphorylation rates. In short, I suggest that in smooth muscle, telokin plays a chaperone role for myosin and its filaments.Key words: smooth muscle, regulation, myosin filament, phosphorylation, activation mechanism, myosin kinase, phosphatase, telokin.