Chemotropism, or directed cell growth in response to a chemical gradient, plays a vital role in many processes, including pollen tube... Show moreChemotropism, or directed cell growth in response to a chemical gradient, plays a vital role in many processes, including pollen tube formation, axon guidance, angiogenesis, and fungal infections. One of the best-studied models of eukaryotic directional sensing is Saccharomyces cerevisiae, also known as budding yeast. Chemotropic growth relies on both pheromone receptor and its associated G-protein. In vegetative yeast cells, the pheromone receptor is uniformly distributed on the plasma membrane. Upon receptor activation, G~y is released from Ga and G~ is subsequently phosphorylated at multiple residues. Free G~y signals to the nucleus via a MAPK cascade that causes G1 cell cycle arrest and induction of mating specific genes. In addition, G~y recruits various polarity proteins that orient actin-directed secretion to a specific growth site, and newly synthesized receptors are delivered to it. This pheromone-induced receptor polarization appears as "crescents" where the mating projection later forms. However, how yeast cell interpret the external signals and establish the chemotropic growth site remains unknown. Here, we characterized the role of Pcl1, which is a cyclin for the CDK Ph085, and which has been implicated in polarization during budding. It is known that G~ phosphorylation plays a role in receptor polarization and chemotropism. Because G~ has a consensus phosphorylation motif for Ph085, and because G~ and Pcl1 showed a genetic interaction, we hypothesized that Ph085-Pcl1 regulates receptor polarity through G~ phosphorylation. Our results showed Pcl1 localized towards mating projection during mating response. Moreover, Pcl1 and G~ exhibit a direct interaction in response to pheromone. Lastly Ph085 inactivation also appears to affect receptor polarity. Hence, our data are consistent with the hypothesis that Pho85-Pcl1 facilitates receptor polarity establishment by phosphorylating G~. M.S. in Biology, May 2015 Show less
