Phosphorylation-dependent and ATP-induced changes in structural array in gizzard myosin filament bundles.

We observed the fine structure of myosin filament bundles in 10 mM MgCl2 and studied the effects of light chain phosphorylation on it by electron microscopy. In the filament bundles of dephosphorylated myosin in the presence of ATP, we found a striation pattern of 13.4 nm periodicity running perpendicularly to the long axis of the filaments. However, we did not observe such a pattern in filament bundles of phosphorylated myosin even when exposed to ATP, but many irregularly arranged projections were seen on their surface. The 13.4 nm period of the striation was so close to the dimension of myosin heads that it seems likely that the striation pattern represents the regular array of myosin heads, which is a consequence of conformational changes in dephosphorylated myosin molecules upon reacting with ATP.