SPARE THE ROD, SPOIL THE REGULATION - NECESSITY FOR A MYOSIN ROD

Regulation of a variety of cellular contractile events requires that v ertebrate smooth and non-muscle myosin ii can achieve an ''off'' state , To examine the role of the myosin rod in this process, we determined the minimal size at which a myosin molecule is capable of regulation via light chain phosphorylation. Expressed smooth muscle myosin subfra gments with as many as 100 amino acids of the coiled-coil rod sequence did not dimerize and were active independently of phosphorylation. To test whether dimerization per se restores regulation of ATPase activi ty mutants were expressed with varying lengths of rod sequence, follow ed by C-terminal leucine zippers to stabilize the coiled-coil. Dimeriz ation restored partial regulation, but the presence of a length of rod approximately equal to the myosin head was necessary to achieve a com pletely off state, partially regulated short dimers could be converted into fully regulated molecules by addition of native rod sequence aft er the zipper, These results suggest that the myosin rod mediates spec ific interactions with the head that are required to obtain the comple tely inactive state of vertebrate smooth and non-muscle myosins, if th ese interactions are prohibited under cellular conditions, unphosphory lated crossbridges can slowly cycle.