We describe a new member of the receptor protein tyrosine phosphatase family, R-PTP-K. cDNA cloning predicts that R-PTP-K is synthesized from a precursor protein of 1,457 amino acids. Its intracellular domain displays the classical tandemly repeated protein tyrosine phosphatase homology, separated from the transmembrane segment by an uncharacteristically large juxtamembrane region. The extracellular domain of the R-PTP-K precursor protein contains an immunoglobulin-like domain and four fibronectin type III-like repeats, preceded by a signal peptide and a region of about 150 amino acids with similarity to the Xenopus A5 antigen, a putative neuronal recognition molecule (S. Takagi, T. Hsrata, K. Agata, M. Mochii, G. Eguchi, and H. Fujisawa, Neuron 7:295-307, 1991). Antibodies directed against the intra-and extracellular domains reveal that the R-PTP-k precursor protein undergoes proteolytic processing, following which both cleavage products remain associated. By site-directed mutagenesis, the likely cleavage site was shown to be a consensus sequence for cleavage by the processing endopeptidase furin, located in the fourth fibronectin type III-like repeat. In situ hybridization analysis indicates that expression of R-PTP-K in the central nervous system is developmentally regulated, with highest expression seen in actively developing areas and, in the adult, in areas capable of developmental plasticity such as the hippocampal formation and cerebral cortex. The mouse R-PTP-K gene maps to chromosome 10, at approximately 21 centimorgans from the centromere.Tyrosine phosphorylation of proteins is involved in an increasing number of cellular signalling events. It was originally implicated in signalling by paracrine-or autocrineacting growth factors and by endocrine hormones such as insulin (see reference 51 for a review). It is now clear that this posttranslational modification is also involved in diverse processes such as the activation of cells of the immune system by antigens (19), signalling by lymphokines (13, 25), and cellular differentiation and survival (10,35,46). In view of the diversity of processes in which tyrosine phosphorylation is involved, it is not surprising that links are also emerging with the process of cell adhesion and cell-cell contact. The Src protein has long been known to be concentrated in focal contacts of fibroblasts (31), and intercellular adhesion junctions have been shown to correspond to sites of increased tyrosine phosphorylation and to be subject to its control (48). Attachment of cells to a fibronectin substrate leads to activation of a tyrosine kinase designated Fak (12). Moreover, it has been shown that cross-linking of cell adhesion molecules leads to changes in tyrosine phosphorylation (1).Most of the processes in which tyrosine phosphorylation is implicated involve the transduction of a signal through the cell membrane. In its best-understood fashion, this can occur through dimerization-mediated activation of members of the receptor tyrosine kinase family by soluble ligands (re...