Hydrophilic and poorly conserved SPX (SYG1, Pho81 and XPR11) domains, of about 180 residues, are found at the N-termini of various proteins, particularly signal transduction proteins. There are proteins harboring the SPX domain in all major eukaryotic kingdoms. SPX domains are composed of three subdomains, which are separated by two areas of low similarity. They are often (1) variable in sequence, (2) split due to insertions, or (3) fragmented due to deletions. The tripartite SPX domain could function as a sensor for inorganic phosphate (Pi) level and is most likely a domain for protein interaction. Possibly all SPX domains bind G-protein β-subunits for the purpose of signal transduction [1,2,3].

Some proteins known to contain a SPX domain are listed below:

• Yeast SYG1, a protein involved in the mating pheromone signal transduction pathway.
• Yeast PHO81, the putative sensor of Pi levels. It is a cyclin-dependent kinase (CDK) inhibitor that is induced by Pi starvation, interacting with cyclin PHO80 to repress the activity of CDK PHO85, thus promoting the expression of PHO2/PHO4 and enhancing yeast tolerance to Pi starvation.
• Vertebrate xenotropic and polytropic retrovirus receptor 1 receptor (XPR1), initially identified as a retrovirus receptor but for which the function in uninfected cells remains unknown.
• Arabidopsis thaliana PHO1, a protein involved in the loading of inorganic phosphate into the xylem of roots. It harbors both aSPX and EXS (see <PDOC51380>) domains.
• Arabidopsis thaliana PHO1-like proteins (identified as PHO1;H1 to PHO1;H10).
• Arabidopsis thaliana SHORT HYPOCOTYL UNDER BLUE 1 (SHB1), homologous to yeast SYG1. It acts in cryptochrome signaling and possible phytochrome- mediated light responses.
• Arabidopsis thaliana AtSPX1-AtSPX4, which are part of the phosphate- signaling network that involves SIZ1/PHR1.
• Barley IDS4 (iron-deficiency specific clone 4). It contains part of the SPX domain and is preferentially expressed in Fe-deficient roots.
• Tomato IDS4-like protein. It interacts with the leucine zipper domain (see <PDOC00029>) of a hypoxia-induced transcription factor involved in the low- oxygen response.

The profile we developed covers the entire SPX domain.