Yeast cells rely on the SPS-sensing pathway to respond to extracellular amino acids. This nutrient-induced signal transduction pathway regulates gene expression by controlling the activity of two redundant transcription factors: Stp1 and Stp2. These factors are synthesized as latent cytoplasmic proteins with N-terminal regulatory domains. Upon induction by extracellular amino acids, the plasma membrane SPS-sensor catalyses an endoproteolytic processing event that cleaves away the regulatory N-terminal domains. The shorter forms of Stp1 and Stp2 efficiently target to the nucleus, where they bind and activate transcription of selected genes encoding a subset of amino acid permeases that function at the plasma membrane to catalyse the transport of amino acids into cells. In the present article, the current understanding of events in the SPS-sensing pathway that enable external amino acids to induce their own uptake are reviewed with a focus on two key issues: (i) the maintenance of Stp1 and Stp2 latency in the absence of amino acid induction; and (ii) the amino-acid-induced SPS-sensor-mediated proteolytic cleavage of Stp1 and Stp2.
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