The budding yeast Saccharomyces cerevisiae has been extensively used as model to study the molecular mechanisms of actin regulation.¹ Filamentous actin (F-actin) in S. cerevisiae assembles into three types of structures, namely actin patches, actomyosin rings and actin cables. Actin patches are bouquets of branched filaments nucleated by the Arp2/3 complexes. Together with Myosin I this branched actin is thought to generate the forces required for the first steps of endocytosis.² Actomyosin rings assemble transiently at the bud neck after mitotic exit. In these rings, bundles of actin filaments together with Myosin II provide the constricting force for cytokinesis.²

Using total internal reflection microscopy (TIRFM), we recently performed an extensive analysis of actin cable dynamics in S. cerevisiae.¹⁶ We found that cables were reorganized throughout the cell cycle via a combination of growth, shrinkage, bending, bundling and translational motility.^16,17 In contrast to previously published results,^14,15 cable extension and movement occurred at a wide range of speeds between 0.5 and 5 µm/s. To our surprise, cable velocity and turnover were highest in unpolarized G₁ cells, and reduced upon cell polarization. To determine the molecular basis for cable dynamics we analyzed cable dynamics in various mutants of actin regulators and polarisome components. We found that the formin Bni1 was associated with cable extension at 1–2 µm/s, while the other formin Bnr1 mediated slower cable extension at rates below 1 µm/s. In addition, we could show that the type V myosin Myo2 drove rapid translational movement of cables along the cell periphery at speeds above 2 µm/s.

Type V myosins are well known for their role in polarized organelle transport.^1,10–12 We now found that plasma membrane-bound Myosin V also directly mediate motility of actin cables. Such a behavior has been repeatedly observed from in vitro gliding assays but has not been demonstrated in vivo, although various myosins have been proposed to move actin filaments within cells.^20,21 The dual function of Myo2 in generating forces for polarized transport as well as actin reorganization is perfectly suited to cell cycle specific requirements of yeast. In unpolarized G₁ cells the cell periphery is covered with randomly oriented cables generated by cortical Bni1. The faster elongation rate and low affinity of Bni1 to filament ends very likely leads to frequent detachment of shorter cables. These can then immediately be captured and moved by cortical Myo2 (Fig. 1). An intriguing possibility that still remains to be tested is that Myo2 may actively pull on Bni1 attached actin filaments and thereby either enhance or terminate the filament elongation. The combination of rapid polymerization and fast motility generates a highly dynamic and randomized network of actin cables, thereby maintaining the unpolarized state of G₁ cells and supporting isotropic growth. In polarized cells most cables in mother cells are nucleated by the bud neck-associated formin Bnr1.^16,22 While Bnr1 has a slower elongation rate than Bni1, it has much higher affinity to filament barbed ends and therefore remains tightly associated with cable ends.¹⁸ A cortical Myo2 motor that encounters such a stably anchored cable will very likely not be able to detach it from the bud neck. Instead, the anchorage of Myo2 to the cortex could be dislodged and myosin motors become free to support polarized organelle transport toward the bud neck.

Cell polarization requires most actin cables to be oriented with their barbed ends pointing toward the polarization site. Paradoxically, cortically anchored Bni1 and Myosin V should both generate forces toward the pointed ends of actin filaments. Therefore, Bni1 and Myo2, both required for proper cell polarization, also seem to antagonize it by randomization of F-actin. Why then, do cells expend so much energy to generate randomness, although they all eventually commit to polarized growth? One reason could be that in S. cerevisiae the decision for a polarization site must be made within an extremely narrow time window with high accuracy.^24–26 This requires the cell to balance the need for rapid decision making with the need for error detection and correction. A highly dynamic network of actin cables serves both ends. On the one hand, it allows the cell to rapidly respond to a polarization cue, since the reorganization of entire cable network only takes several seconds in G₁ cells.^16,17 On the other hand, randomization of cables by Bni1 and Myo2 prevents the cell from falling into kinetically trapped states, such as the formation of multiple polarization foci. Indeed, when cable dynamics were forced to slow down in cells already defective in Rho GTPase recycling, a significant proportion of them formed double polarization caps.¹⁶ This result suggests that cells evolved a coupled mechanism of GTPase recycling and actin cable dynamics to ensure the fidelity of cell polarization.