In order to pass through Start, and commit to a new round of cell division, the yeast cell must reach a critical cell size and also sense other signals such as the presence of adequate nutrients and mating pheromones. Start is not a decision to be taken lightly; the early events that ensue once Start is passed include DNA replication and a dramatic reorganization of the actin cytoskeleton that allows the polarized cell growth necessary for bud emergence. We are exploring the regulatory pathways that coordinate these important events early in the cell cycle.

Our efforts to understand the functions of Pho85 cyclin-dependent protein kinases (see Cdks and individual project descriptions) have led us to explore protein complexes that regulate the actin cytoskeleton and cell polarity during the cell cycle. We discovered that an actin cytoskeletal protein called Rvs167 is a target of G1-specific Pho85 Cdks - this discovery identified a connection between Cdks and the actin cytoskeleton and highlighted a role for Pho85 and Rvs167 in cell morphogenesis and polarity. Our work also suggests that the Cdk-actin connection is functionally conserved in mammalian cells - we used complementation experiments in yeast and mammalian cells to show that mammalian Cdks5 and Pho85 are functional homologues. Cdk5 is activated by a neuronal-specific regulation, p35, to control neuronal migration and neurite outgrowth. Like Pho85, p35-Cdk5 regulates actin cytoskeletal dynamics, likely through phosphorylation of actin regulatory proteins. The Pho85 target, Rvs167, is similar to mammalian amphiphysins which are neuronal specific proteins involved in endocytosis at nerve termini. Thus, we suspect that our studies of the Pho85-Rvs167 pathway in yeast will inform us about the function and regulation of Cdk5 in neuronal cells. We are currently using a variety of approaches to explore the role of Pho85, Rvs167 and other actin regulatory proteins in coordinating cytoskeletal rearrangements and cell proliferation.

 

Filamentous actin structures during the cell cycle. Cortical actin patches are represented by dots and cytoplasmic actin cables are represented by lines.  During G1- and S-phases, actin patches are polarized to the growing bud and actin cables are oriented in this direction.  After G2-phase, actin patches and cables are asymmetrically distributed between mother and bud. At M-phase, actin patches and cables localize to and  are oriented toward the bud neck, respectively.

Wild-type yeast strains during mid-log phase, fixed, and stained with rhodamine-conjugated phalloidin.  Actin is visualized using rhodamine fluorescence optics.