Phosphorylation of smooth muscle myosin heads regulates the head-induced movement of tropomyosin

It has been shown that skeletal and smooth muscle myosin heads binding to a ctin results in the movement of smooth muscle tropomyosin, as revealed by a change in fluorescence resonance energy transfer between a fluorescence do nor on tropomyosin and an acceptor on actin (Graceffa, P. (1999) Biochemist ry 38, 11984-11992), In this work, tropomyosin movement was similarly monit ored as a function of unphosphorylated and phosphorylated smooth muscle myo sin double-headed fragment smHMM. In the absence of nucleotide and at low m yosin head/actin ratios, only phosphorylated heads induced a change in ener gy transfer. In the presence of ADP, the effect of head phosphorylation was even more dramatic, in that at all levels of myosin head/actin, phosphoryl ation was necessary to affect energy transfer, It is proposed that the regu lation of tropomyosin position on actin by phosphorylation of myosin heads plays a key role in the regulation of smooth muscle contraction. In contras t, actin-bound caldesmon was not moved by myosin heads at low head/actin ra tios, as uncovered by fluorescence resonance energy transfer and disulfide cross-linking between caldesmon and actin, At higher head concentration cal desmon was dissociated from actin, consistent with the multiple binding mod el for the binding of caldesmon and myosin heads to actin (Chen, Y., and Ch alovich, J. M. (1992) Biophys. J. 63, 1063-1070).