This thesis describes studies aimed at elucidating the molecular mechanisms that regulate the SPS (Ssy1-Ptr3-Ssy5) signal transduction pathway in the yeast Saccharomyces cerevisiae. This pathway is induced by extracellular amino acids and facilitates their uptake. The most downstream effectors of the SPS pathway, the homologous transcription factors Stp1 and Stp2 (Stp1/2), are synthesized as latent precursors with N-terminal regulatory domains that restrict their nuclear accumulation. Amino acid-induced signaling, initiated by the plasma membrane localized receptor Ssy1, leads via Ptr3 to the activation of the endoprotease Ssy5. Active Ssy5 cleaves the regulatory domains in Stp1/2. As a consequence, the processed transcription factors lacking their N-terminal domains accumulate in the nucleus and activate the transcription of amino acid permease genes to enhance the uptake capacity of cells.

Ssy5 is synthesized as a zymogen precursor that processes itself into a prodomain and catalytic (Cat) domain that remain non-covalently associated. We found that the prodomain functions as an inhibitor of the Cat domain. Signaling triggers the degradation of the prodomain by the proteasome, thereby releasing Cat domain activity (paper I). We identified a motif in the prodomain that functions as inducible phosphodegron. Upon signaling, this motif is phosphorylated which triggers prodomain polyubiquitylation, and as a consequence, its proteasomal degradation (paper II). Also, we found that Ptr3 functions to mediate prodomain phosphorylation upon signaling and that protein phosphatase 2A constitutively mutes phosphorylation-dependent activation of Ssy5 (paper III).

Finally, in addition to the regulation of the processing protease Ssy5, the control of transcriptional activity of Stp1 depends on a motif within its N-terminal regulatory domain, designated Region I. We found that Region I mediates latency by functioning as cytoplasmic retention determinant and nuclear degron (paper IV).