MYOSIN ESSENTIAL LIGHT-CHAIN ISOFORMS MODULATE THE VELOCITY OF SHORTENING PROPELLED BY NONPHOSPHORYLATED CROSS-BRIDGES

The differential effects of essential light chain isoforms (LC17a and LC17b) on the mechanical properties of smooth muscle were determined b y exchanging recombinant for endogenous LC17 in permeabilized smooth m uscle treated with trifluoperazine (TFP). Go-precipitation with endoge nous myosin heavy chain verified that 40-60% of endogenous LC17a could be exchanged for recombinant LC17a or LC17b. Upon addition of MgATP i n Ca2+-free solution, recombinant LC17 exchange induced slow contracti ons unaccompanied by regulatory light chain (RLC) phosphorylation only in TFP-treated, but not in untreated, permeabilized smooth muscle; th e shortening velocity and rate of force development were approximately 1.5 and 2 times faster, respectively, in response to LC17a than LC17b . Additional incubation with recombinant, thiophosphorylated RLC incre ased the shortening velocity, independent of the LC17 isoform exchange d. The LC17-induced contractions of TFP-treated muscles were abolished by prior addition of nonphosphorylated RLC. We suggest that LC17 stif fens the lever arm of myosin and, in the absence of regulation by RLC, permits cross-bridge cycling without requiring RLC phosphorylation. O ur results are compatible with nonphosphorylated RLC acting as a repre ssor and with LC17 isoforms modulating the MgADP affinity and, consequ ently, rate of cooperative cycling of nonphosphorylated cross-bridges.