Filament formation in smooth muscle homogenates.

To provide more detailed information on the aggregation properties of smooth muscle myosin, we have extended earlier work on the formation of thick filaments when homogenates of guinea-pig taenia coli and chicken gizzard muscle are diluted. In both preparations there is a slow and a fast phase of filament formation. The slow phase, which generally develops over several hours, appears to depend primarily on the ATP concentration while the rapid phase, which develops over 5-15 min, is influenced by the extent of dilution, homogenization conditions, divalent cation concentration, ATP concentration and presence of chicken gizzard tropomyosin. Many of these effects on the rapid phase can be explained by postulating that filament formation only takes place when the ATP concentration is reduced. There are significant differences between the filament populations formed from each muscle, with those from taenia coli being shorter than those from gizzard. Two types of filament are present in preparations from each muscle, the first being characterized by the presence of a central bare zone and cross striations at both ends, whilst the second have cross striations along their entire length; the periodicity of the cross striations appears to be 14.5 nm. The bare zone filaments have an average length and width of 325 nm and 17.6 nm respectively, while the corresponding values for the cross striated filaments is 3 : 1 for taenia coli and 1 : 3 for chicken gizzard, which accounts for the difference in average filament length observed between these preparations. The gizzard filaments appear to form more readily than those of taenia coli.