SKELETAL-MUSCLE MYOSIN MONOMER IN EQUILIBRIUM WITH FILAMENTS FORMS A FOLDED CONFORMATION

Rabbit skeletal myosin forms stable filaments under physiological cond itions, and only a small amount stays as a monomer in equilibrium with filaments. The myosin monomers were observed in two conformational st ates, as extended and folded forms upon electron microscopy and gel fi ltration high performance liquid chromatography, The fraction of monom ers in the folded conformation increased with a decrease in the concen tration of NaCl below 0.2 M, and the conformational state was affected neither by the presence of ATP nor by the phosphorylation of regulato ry light chain. In most of the folded monomers, the tail bent back tow ard the heads at one region, 45 nm apart from the head-tail junction, and the remaining tail portion containing the C-terminal tip appeared to interact with the head-hail junction., Only a small percentage of t he folded monomers was in a more compact conformation close to the 10 S conformation of vertebrate smooth muscle and non-muscle myosins, The folded monomers, however, may not trap the products of ATP hydrolysis as assessed by single turnover experiments. The percentage of monomer s in the 10 S-like conformation was increased by the exchange of a reg ulatory light chain with the smooth muscle light chain, indicating the participation of head-tail junction, including the regulatory light c hain in the formation of folded conformation. The folded conformation may be common to various myosin IIs, suggestive of common roles for th e folded monomers.