Regulation of cell shape in Euglena gracilis. IV. Localization of actin, myosin and calmodulin.

The immunofluorescence patterns for actin, myosin, calmodulin and tubulin were observed in Euglena gracilis Klebs strain Z during the biological clock-controlled shape changes observed with division-synchronized cells, and during two shock responses that induce cell rounding. The fluorescence patterns for actin, myosin, calmodulin and tubulin show a high degree of coincidence and are visualized as lines running parallel to, and having the same spacing as, the pellicle strips beneath the plasma membrane. The fluorescence patterns remain intact during the daily shape changes, implying that the shape changes do not result from cycles of polymerization and depolymerization of the microtubules and microfilaments. Resuspension of cells in Ca2+-free medium induces cell rounding of many of the cells. The actin and calmodulin patterns are partially disrupted by the Ca2+-free resuspension, while the myosin pattern is almost totally disrupted. Microtubules are unaffected by this treatment. Prior exposure of cells to the calmodulin antagonist trifluoperazine or to the microfilament-stabilizing peptide phalloidin stabilize the actin, myosin and calmodulin patterns against disruption by the Ca2+-free resuspension and other shock responses. The possibility of an actomyosin contractile system controlled by calmodulin is discussed.