The protein kinase C-MAP kinase pathway of Saccharomyces cerevisiae responds to stresses to the cell surface (e.g., caused by heat shock) via two plasma membrane sensors, Hcs77 and Mid2, and to aberrations in the actin cytoskeleton {e.g., caused by latrunculin treatment) by a different, unknown mechanism. Protein kinase C (encoded by PKC1) in part regulates a MAP kinase, Mpkl. Mpt5 is a yeast Pumilio homologue whose in vivo role is unclear. In this thesis I present evidence that Mpt5 is a novel regulator of the Pkcl pathway. Mutants lacking Mpt5 (in three different strain backgrounds) display phenotypes characteristic of mutants defective in the Pkcl pathway. These results are consistent with Mpt5 acting as a partial activator of the Pkcl pathway. Extensive genetic analyses indicate that Mpt5 acts in the Pkcl pathway itself, rather in a pathway parallel to this of related function. Epistatic analysis indicates that Mpt5 acts upstream of Pkc1. Mutants lacking Mpt5 are defective in activation of the Pkcl pathway. Heat shock- induced activation of Mpkl (dependent on Mid2 and Pkcl) is compromised in mptSA mutants as determined directly by analysing the phosphorylation state of Mpkl and indirectly by assaying acquired thermotolerance. Loss of Mpt5 prevents proper heat shock-induced depolarisation of the actin cytoskeleton (dependent on Hcs77 and Pkcl, but not Mpkl) but does not affect the ability of activated Pkcl to depolarise actin. Loss of Mpt5 does not affect activation of Mpkl by latrunculin treatment. Thus, Mpt5 specifically acts between the cell surface sensors and Pkcl and is required for proper activation of the kinase in response to cell surface stresses. It is known that Mpt5 activity is required for normal cellular lifespan. I present evidence indicating that activity of Pkcl is limiting for lifespan in mptSA mutants. The Pkcl pathway may thus regulate cell ageing.