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* Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3202; and We report that the actin assembly inhibitor
latrunculin-A (LAT-A) causes complete disruption of
the yeast actin cytoskeleton within 2-5 min, suggesting
that although yeast are nonmotile, their actin filaments
undergo rapid cycles of assembly and disassembly in
vivo. Differences in the LAT-A sensitivities of strains
carrying mutations in components of the actin cytoskeleton suggest that tropomyosin, fimbrin, capping protein, Sla2p, and Srv2p act to increase actin cytoskeleton
stability, while End3p and Sla1p act to decrease stability. Identification of three LAT-A resistant actin mutants demonstrated that in vivo effects of LAT-A are
due specifically to impairment of actin function and implicated a region on the three-dimensional actin structure as the LAT-A binding site.
LAT-A was used to determine which of 19 different
proteins implicated in cell polarity development require actin to achieve polarized localization. Results
show that at least two molecular pathways, one actindependent and the other actin-independent, underlie
polarity development. The actin-dependent pathway
localizes secretory vesicles and a putative vesicle docking complex to sites of cell surface growth, providing an
explanation for the dependence of polarized cell surface growth on actin function. Unexpectedly, several
proteins that function with actin during cell polarity development, including an unconventional myosin
(Myo2p), calmodulin, and an actin-interacting protein
(Bud6/Aip3p), achieved polarized localization by an actin-independent pathway, revealing interdependence among cell polarity pathways. Finally, transient actin
depolymerization caused many cells to abandon one
bud site or mating projection and to initiate growth at a
second site. Thus, actin filaments are also required for
maintenance of an axis of cell polarity.
Department of
Chemistry and Biochemistry, University of California, Santa Cruz, California 95064
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