The p21-activated kinases Cla4 and Ste20 regulate vacuole inheritance in S. cerevisiae.
Bartholomew, Clinton Ron
Each time budding yeast divide they ensure that both the mother and daughter cell inherit a vacuole. Because budding yeast divide asymmetrically by budding, the vacuole must be actively transported into the bud. As the mother cell begins budding, a tubular and vesicular segregation structure forms which is transported into the bud by the myosin V motor, Myo2, bound to the vacuole-specific myosin receptor Vac17. Upon arriving in the bud the segregation structure is resolved to found the daughter vacuole. The mechanism that regulates segregation structure resolution in a spatially dependent manner is unknown. Directionality in vacuole transport is ensured by the bud-specific degradation of Vac17. It has been proposed that bud-specific degradation of Vac17 is promoted by proteins localized to, or activated solely in, the bud. The p21-activated kinases (PAK) Cla4 and Ste20 are localized to and activated in the bud. Here I report that Cla4 localized to the segregation structure just prior to segregation structure resolution. Cells lacking PAK function failed to resolve the segregation structure. Overexpression of either Cla4 or Ste20 inhibited vacuole inheritance and this inhibition was suppressed by the expression of non-degradable VAC17. Finally, PAK activity was required for Vac17 degradation in late-M and CLA4 overexpression promoted Vac17 degradation. I propose that Cla4 and Ste20 are bud-specific proteins that promote segregation structure resolution and degradation of Vac17.