Amino acid sensor 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. Amino-acid permease homolog that seems to interact directly with the extracellular signaling molecules, but has no amino acid transporter activity. May recruit casein kinases YCK1 and YCK2 to hyperphosphorylate and activate downstream component PTR3 in response to amino acid stimulus. Component of the plasma membrane SPS (SSY1-PTR3-SSY5) amino acid sensor complex. Interacts directly with PTR3 and SSY5. Interacts with YCK1. Belongs to the amino acid-polyamine-organocation (APC) superfamily.