FIG. 6.

Proposed model for Mpt5p action. Mpt5p affects cell integrity and stress tolerance predominantly, if not exclusively, by modulating CWI-Pkc1p activity. Mpt5p affects Pkc1p activity in part by directly binding to and inhibiting the stability of the LRG1 transcript. The reduction in Lrg1p levels, a Rho1-GAP, results in increased amounts of active Rho1p-GTP complex. This complex binds to and activates Pkc1p, which then promotes cell integrity and stress tolerance (27). Mpt5p must also affect the CWI-Pkc1p pathway independently of LRG1, by binding to one or more additional and unidentified transcripts whose products act directly or indirectly in the pathway. The effects of Mpt5p on chronological and replicative life span are only partly dependent on CWI-Pkc1p signaling. Other, as-yet-unknown target transcripts likely regulate other distinct pathways that, with the CWI pathway, cooperate to regulate cellular life span.