Tropomyosin promotes myosin-V motility along actin cables. Accumulation of fimbrin at actin patches relieves Myo1p from tropomyosin-mediated inhibition, ensuring maximal myosin-I motor activity at these sites. Thus, spatial regulation of myosin motor function is in part controlled by specific changes in the composition of the actin track.
SummaryFission yeast myosin-I (Myo1p) not only associates with calmodulin, but also employs a second light chain called Cam2p. cam2 cells exhibit defects in cell polarity and growth consistent with a loss of Myo1p function. Loss of Cam2p leads to a reduction in Myo1p levels at endocytic patches and a 50% drop in the rates of Myo1p-driven actin filament motility. Thus, Cam2p plays a significant role in Myo1p function. However, further studies indicated the existence of an additional Cam2p-binding partner. Cam2p was still present at cortical patches in myo1 cells (or in myo1-IQ2 mutants, which lack an intact Cam2p-binding motif), whereas a cam2 null (cam2) suppressed cytokinesis defects of an essential light chain (ELC) mutant known to be impaired in binding to PI 4-kinase (Pik1p). Binding studies revealed that Cam2p and the ELC compete for Pik1p. Cortical localization of Cam2p in the myo1 background relied on its association with Pik1p, whereas overexpression studies indicated that Cam2p, in turn, contributes to Pik1p function. The fact that the Myo1p-associated defects of a cam2 mutant are more potent than those of a myo1-IQ2 mutant suggests that myosin light chains can contribute to actomyosin function both directly and indirectly (via phospholipid synthesis at sites of polarized growth).
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