Protease component of the SPS-sensor system, which regulates the expression of several amino acid-metabolizing enzymes and amino acid- and peptide-permeases in response to extracellular amino acid levels by controlling the activity of two transcription factors, STP1 and STP2. Catalyzes the activation of these transcription factors, which are synthesized as latent cytoplasmic precursors, by proteolytic removal of an N-terminal inhibitory domain containing cytoplasmic retention motifs. SSY5 binds as an inactive protease complex to STP1. In response to extracellular amino acids and dependent on the other SPS-sensor components, the inhibitory propeptide is induced to dissociate, and thereby enables the catalytic domain to process STP1. Component of the plasma membrane SPS (SSY1-PTR3-SSY5) amino acid sensor complex. Down-regulated after extracellular amino-acid addition. The propeptide is autoproteolytically cleaved from the catalytic domain but remains associated, forming an inactive protease complex. This processing occurs even in the absence of signaling. Belongs to the peptidase S64 family.