Systematic Analysis of Yeast F‐box Proteins Reveals a New Role of Ubiquitination in Polarity Establishment

Abstract

Ubiquitination is a post‐translational modification that regulates protein activity by tagging specific substrates for degradation. Regulation of proteins by ubiquitination has been demonstrated to help terminate the response to mating pheromone in budding yeast. The yeast pheromone response is comprised of two branches, one leading to MAPK activation and gene transcription, the other leading to cellular polarization and shmoo formation. The Skp1/Cdc53/F‐box (SCF) ubiquitin ligase employs different F‐box subunits to recruit specific substrates for ubiquitination. In yeast, F‐box proteins have been identified as interchangeable components of the SCF complex. To identify pathway regulators, we conducted a screen in which we measured activation of the pheromone response in 16 yeast strains ‐‐ each lacking one of the candidate F‐box genes. We characterized the abilities of these deletion mutants to affect either the MAPK or the polarity branch of the pathway. Our study reveals that one F‐box protein, Pfu1, selectively regulates the polarity branch. Using live cell imaging in a microfluidic chamber, we demonstrate that Pfu1 is required to restrict the cell to a single polarity site. In contrast, Pfu1 is has no effect on MAP kinase activation or transcriptional induction. While polar axis formation is a fundamental and well characterized feature of budding, the mechanisms used to regulate cell polarity components during mating are largely unknown. The existence of an F‐box protein exclusively involved in mating projection development reveals ubiquitination as a means to independently regulate branches of a signal transduction pathway.