The hair cell antigen/PTPRQ: lipid phosphatase activity, intracellular domain interactors, apical targeting and evidence that it is a proteoglycan

Ptprq, a receptor-like phosphatase known to be associated with the shaft connectors of stereocilia, is a protein required for the maintenance of the hair-bundle structure. The extracellular domain of Ptprq has numerous potential sites for N-glycosylation and the cytoplasmic domain has phosphatidyl inositol phosphatase activity in vitro. The current project was aimed at determining whether Ptprq regulates inositol phosphate levels in the hair bundle, identifying the intracellular binding partners of Ptprq, understanding what targets Ptprq to the apical membrane, and finally, elucidating whether Ptprq is a proteoglycan. The results show that EHD3, a protein involved in endocytosis, interacts with the intracellular domain of Ptprq and that a major part of the apical targeting signal in Ptprq lies in the N-glycosylated moieties of the extracellular domain. Also, evidence was found indicating that Ptprq is a chondroitin sulphate proteoglycan and that there may be a developmentally-regulated isoform carrying the DSD-1 epitope. Glycosylation variants of Ptprq may be responsible for the regional variation in the appearance of the shaft connectors.