Involvement of 17K Dalton Light Chain of Smooth Muscle Myosin in Substrate-Induced Conformational Change

Abstract

Conformational changes of the region involving 17K dalton light chain (G₂) in gizzard myosin induced by ATP and its analogs were studied by a combination of light chain exchange and fluorescence spectroscopy. The cysteinyl residues of myosin light chain mixture, 20K dalton G₁ and 17K dalton G₂ chain, were labeled with a fluorescent thiol reagent, N-(1-anilinonaphthyl-4)maleimide (ANM). Chicken gizzard myosin was incubated with a large excess of the fluorescence labeled exogenous light chains in 2 M urea. After removal of urea and unbound light chains, a strong fluorescent band was observed at the position of the 17K dalton light chain (G₂) of myosin on SDS PAGE. There was no difference in enzymatic activity and light chain composition between myosins before and after light chain exchange. The emission spectrum of ANM label in 17K dalton light chain in the bound state in myosin showed a fluorescence enhancement and a blue shift on adding ATP but not ADP. The spectral change disappeared after conversion of ATP to ADP. Various nucleoside triphosphates other than ATP had similar effects on the fluorescence spectrum. The relative effectiveness of most nucleotides on the fluorescence spectral change of ANM in the light chain bound in myosin agreed well with that on tryptophan residues in myosin, except in the case of ADP and CTP. These results suggest that 17K dalton light chain of gizzard myosin is closely related to the ATPase site of myosin.