May play a role in phosphate homeostasis. Binds inositol hexakisphosphate (Ins6P) and similar inositol polyphosphates, such as 5-diphospho-inositol pentakisphosphate (5-InsP7); these are important intracellular signaling molecules. The SPX domain has high affinity for inositol polyphosphates, such as myo-inositol hexakisphosphate and 5-diphospho-myo-inositol pentakisphosphate (5-InsP7). Its affinity for inorganic phosphate is tow to three orders of magnitude lower. Belongs to the SYG1 (TC 2.A.94) family.