The SPX domain is named after SYG1/Pho81/XPR1 proteins. This 180 residue length domain is found at the amino terminus of a variety of proteins. In the yeast protein SYG1, the N terminus directly binds to the G- protein beta subunit and inhibits transduction of the mating pheromone signal []suggesting that all the members of this family are involved in G-protein associated signal transduction. The C-terminal of these proteins often have an EXS domain () [].The N-termini of several proteins involved in the regulation of phosphate transport, including the putative phosphate level sensors PHO81 from Saccharomyces cerevisiae and NUC-2 from Neurospora crassa, are also members of this family [, ]. NUC-2 contains several ankyrin repeats ().Several members of this family are the XPR1 proteins: the xenotropic and polytropic retrovirus receptor confers susceptibility to infection with Murine leukemia virus (MLV) []. The similarity between SYG1, phosphate regulators and XPR1 sequences has been previously noted, as has the additional similarity to several predicted proteins, of unknown function, from Drosophila melanogaster, Arabidopsis thaliana, Caenorhabditis elegans, Schizosaccharomyces pombe, and Saccharomyces cerevisiae [, ]. In addition, given the similarities between XPR1 and SYG1 and phosphate regulatory proteins, it has been proposed that XPR1 might be involved in G-protein associated signal transduction [, , ]and may itself function as a phosphate sensor [].
