The role played by Ca2+ in the stability of cytoplasmic actin and myosin filaments was investigated ultrastructurally with negatively stained isolated cytoplasm from Chaos carolinensis. Cytoplasm was incubated in solutions containing 5, 10, 15 and 25 mM EGTA for periods of time varying from 2 to 20 min. As either the EGTA concentration or duration of incubation was increased, the extent of myosin and actin filament depolymerization increased. The actin filaments depolymerized except where they were stabilized by interaction with myosin. With longer incubation times or higher EGTA concentrations complete depolymerization of the actin filaments could be accomplished. Myosin aggregates also disassembled and became shorter, while monomeric myosin labelled adjacent thin filaments to form arrowhead complexes resembling myosin enriched actomyosin . These actomyosin complexes were relatively stable at low Ca2+ concentrations. In addition, the complexes showed a characteristic 35 nm periodicity and were dissociable in the presence of Mg2+-ATP. The actin containing filaments were more labile at low Ca2+ concentrations than the myosin aggregates. These results suggest that in cells capable of regulating their Ca2+ concentrations efficiently, filament polymerization-depolymerization could play a role in the control of cytoplasmic streaming.
ASJC Scopus subject areas
- Cell Biology