Helical Order in Tarantula Thick Filaments Requires the “Closed” Conformation of the Myosin Head

Myosin heads are helically ordered on the thick filament surface in relaxed muscle. In mammalian and avian filaments this helical arrangement is dependent on temperature and it has been suggested that helical order is related to ATP hydrolysis by the heads. To test this hypothesis, we have used electron microscopy and image analysis to study the ability and temperature dependence of analogs of ATP and ADP.Pi to induce helical order in tarantula thick filaments. ATP or analogs were added to rigor myofibrils or purified thick filaments at 22 °C and 4 °C and the samples negatively stained. The ADP.Pi analogs ADP.AlF4 and ADP.Vi, and the ATP analogs ADP.BeFx, AMPPNP and ATPγNH2, all induced helical order in tarantula thick filaments, independent of temperature. In the absence of nucleotide, or in the presence of ADP or the ATP analog, ATPγS, there was no helical ordering. According to crystallographic and tryptophan fluorescence studies, all of these analogs, except ATPγS and ADP, induce the “closed” conformation of the myosin head (in which the γ phosphate pocket is closed). We suggest that helical order requires the closed conformation of the myosin head but is not dependent on the hydrolysis of ATP.