Brain-specific receptor-type protein-tyrosine phosphatase RPTP beta is a chondroitin sulfate proteoglycan in vivo

By using polyclonal antiserum, which recognizes multiple proteoglycan core proteins, we isolated a cDNA species for an unknown chondroitin sulfate proteoglycan in bovine brain. Unexpectedly, DNA sequencing revealed that the cDNA encodes an open reading frame highly homologous to the human receptor-type protein-tyrosine phosphatase, RPTP beta. To prove that RPTP beta is a proteoglycan, we raised three polyclonal antibodies against extracellular and cytoplasmic domains of human RPTP beta. These antibodies have been shown to react with a smear band ranging from 350 to 500 kDa in human brain extracts. Digestion with chondroitinase ABC eliminated this smear and gave rise to a 310/300-kDa doublet band that was not detected without digestion, indicating that almost all of the RPTP beta molecules in the brain contain chondroitin sulfate chains. In the cerebellum, immunofluorescence staining of chondroitinase-treated sections revealed pericellular localization of RPTP beta in the external and internal granular layers. These data establish that RPTP beta is expressed constitutively as a chondroitin sulfate proteoglycan in the brain, and suggest that chondroitin sulfates may be an essential component for the physiological function of RPTP beta in vivo.