Using genetic and molecular approaches we have unraveled the basic mechanisms of an intracellular signal transduction pathway that responds to the presence of extracellular amino acids and transmits signals from the plasma membrane to the nucleus to induce the transcription of amino acid permease (AAP) genes. In the plasma membrane (PM) the amino acid receptor Ssy1 functions with two intracellular proteins, Ptr3 and Ssy5, as the fundamental components of the SPS (Ssy1-Ptr3-Ssy5) -sensing pathway.1

 
SPS-sensing pathway in Saccharomyces cerevisiae
 

 
 Figure 1. - The non-induced and induced states of the SPS sensing pathway.

 Left panel - In the absence of inducing amino acids the SPS-sensor is present in the PM in its preactivation conformation (red).1 The transcription factors Stp1 and Stp2 (DNA binding motifs – green boxes) are synthesized as inactive latent precursors that localize to the cytosol due to the presence of a cytoplasmic retention signals (anchor) that prevent the unprocessed full-length forms to efficiently enter the nucleus.2,3 In the absence of the inner nuclear membrane Asi complex (Asi1-Asi2-Asi3), the low levels of full-length Stp1/Stp2 that escape cytoplasmic retention enter the nucleus (dashed arrow) and derepress AAP gene expression.4,5,6 The ability of the Asi complex to prevent transcription is dependent on the presence of the N-terminal regulatory sequences of Stp1/Stp2.7

Right panel - In the presence of inducing amino acids the SPS sensor is activated (green), unfettering the intrinsic proteolytic activity of Ssy5,8,9,10,11 resulting in the endoproteolytic processing (scissors) of Stp1 and Stp2. The shorter activated forms of Stp1 and Stp2, lacking the inhibitory N-terminal domains, are targeted to the nucleus where they bind SPS-sensor regulated promoters (UASaa) and induce transcription.2,3,5,6,7 The increased transcription of AAP genes results in induced rates of amino uptake. AAPs are cotranslationally inserted into the ER membrane, which is contiguous with the outer nuclear membrane. Movement of AAPs to the PM (represented by the dashed arrow) requires the ER membrane-localized chaperone Shr3.12,13 Understanding membrane protein biogenesis is a prerequisite to fully understand the mechanisms involved in nutrient assessments; the receptor component of the extracellular amino acid sensor (Ssy1), which is structurally related to AAPs, requires Shr3 to localize to the plasma membrane. Consequently, cells lacking SHR3 are unable to both sense and take up amino acids.14

Based on our understanding of amino acid sensing mechanisms in yeast, we have examined whether similar mechanisms operate in the human fungal pathogen Candida albicans. Candida possesses all of the SPS-sensor pathway components,15 and our results indicate that C. albicans cells do indeed sense and respond to extracellular amino acids in a manner that is remarkably similar to yeast cells; extracellular amino acids induce the proteolytic processing of the Candida Stp1 and Stp2 homologs, which activates multiple systems necessary for nitrogen source uptake.16 Our analysis indicates that Candida cells lacking the ability to respond to and take up amino acids are unable to efficiently establish virulent infections in a mouse model.15 To facilitate further studies we established experimental conditions that enable wild-type (OrR) Drosophila to serve as model hosts for studying C. albicans virulence.17 Using this fly model we have found that the transcription factor, Stp1, and its upstream activators, Ssy1 and Ssy5, are required for full virulence.

 

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