Proteins containing this domain are a family of phosphoinositide phosphatases with substrates that include phosphatidylinositol-4,5-diphosphate and phosphatidylinositol-3,4,5-trisphosphate. This family is conserved in deuterostomes; VSP was first identified as a sperm flagellar plasma membrane protein in Ciona intestinalis []. Gene duplication events in primates resulted in the presence of paralogs, transmembrane phosphatase with tensin homology (TPTE) and TPTE2, that retain protein domain architecture but, in the case of TPTE, have lost catalytic activity. TPTE, also called cancer/testis antigen 44 (CT44), may play a role in the signal transduction pathways of the endocrine or spermatogenic function of the testis. TPTE2, also called TPTE and PTEN homologous inositol lipid phosphatase (TPIP), occurs in several differentially spliced forms; TPIP alpha displays phosphoinositide 3-phosphatase activity and is localized on the endoplasmic reticulum, while TPIP beta is cytosolic and lacks detectable phosphatase activity [, ]. VSP/TPTE proteins contain an N-terminal voltage sensor consisting of four transmembrane segments, a protein tyrosine phosphatase (PTP)-like phosphoinositide phosphatase catalytic domain, followed by a regulatory C2 domain [].
