This family of fungal proteins is involved in the processing of membrane bound transcription factor Stp1 [1]. The processing causes the signalling domain of Stp1 to be passed to the nucleus where several permease genes are induced. The permeases are important for uptake of amino acids, and processing of tp1 only occurs in an amino acid-rich environment. This family is predicted to be distantly related to the trypsin family (MEROPS:S1) and to have a typical trypsin-like catalytic triad [1].

1: Amino acid signaling in yeast: casein kinase I and the Ssy5 endoprotease are key determinants of endoproteolytic activation of the membrane-bound Stp1 transcription factor. Abdel-Sater F, El Bakkoury M, Urrestarazu A, Vissers S, Andre B; Mol Cell Biol 2004;24:9771-9785. PMID:15509782