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Cdc42 and the polarisome control the timing of multiple outgrowths in yeast.

Many types of cells grow polarized structures of defined sizes, but virtually nothing is known about how a cell starts and stops this process. On page 207, Bidlingmaier and Snyder provide the first report of regulatory proteins involved in this timing control, and find that initiation and termination of polarized growth are controlled by distinct but overlapping sets of proteins.

The authors investigated the growth of mating projections in Saccharomyces cerevisiae. This yeast responds to gradients of pheromone by assuming a shmoo-like shape pointing in the direction of the signal. Shmoos still form in the presence of uniformly high concentrations of pheromone (as may result when cells are closely packed together), but they poke out sequentially in random directions to sample the environment. Bidlingmaier and Snyder found that the time between the initiation of each of these new mating projections varied depending on the relative activity of both Cdc42 and its effectors in the multiprotein, actin-polymerizing polarisome complex. Timing of growth termination was independent of the upstream Cdc42 regulators, but dependent on polarisome components and two proteins implicated in the downstream process of cell fusion.

Pheromone signaling is thought to cause Cdc42 activation that is initially spread over the membrane and gradually localized via a positive feedback loop involving Cdc42 transport. The resultant actin polarization process may deplete a rate-limiting initiation factor such as Cdc42 or pheromone receptor from the membrane, but this factor gradually builds up again to initiate polarization at a new site. Termination of outgrowth at one site is not requisite for reinitiation at a new site, but may supply some factors that increase the rate of reinitiation. ▪