This region has been named the SPX domain after (Syg1, Pho81 and XPR1). The domain is found at the amino terminus of a variety of proteins. 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 plays an important role in the phosphate-regulated signal transduction pathway in N. crassa. It shows high similarity to a cyclin-dependent kinase inhibitory protein Pho81, which is part of the phosphate regulatory cascade in S. cerevisiae. Both, NUC-2 and Pho81, have multi-domain architecture, including the SPX N-terminal domain following by several ankyrin repeats and a putative C-terminal glycerophosphodiester phosphodiesterase domain (GDPD) with unknown function.
Jiyao W et al.(2023). "The conserved domain database in 2023.",