MPTP5, a Putative Murine Homolog of HPTP8, Is Expressed in Specialized Regions of the Brain and in the B-Cell Lineage

regulation of cell activation, growth, and differentiation. To fturther elucidate the fine tuning of cell growth and differentiation through tyrosine phosphorylation, we tried to isolate mouse receptor-type PTP (RPTP) cDNA clones by screening mouse brain cDNA libraries with mouse CD45 PTP domain probes under reduced-stringency conditions. Characterization of isolated cDNA clones for RPTP showed that the cytoplasmic region contains two tandem repeats of PTP domain of about 230 amino acids with intrinsic phosphatase activity. The extracellular region was composed of immunoglobulin (Ig)-like domains and fibronectin type m (FN-m)-like domains. The gene was highly homologous to human PTP8 (HPTP8) and thus was named MPTP & (murine counterpart of HPTP8). The MPTPB gene appeared to generate at least three species of mRNA, which differ in the composition of the extracellular domain: type A, one Ig-like and four FN-III-like domains; type B, on