A Human Transmembrane Protein-Tyrosine-Phosphatase, PTPζ, is Expressed in Brain and has an N-Terminal Receptor Domain Homologous to Carbonic Anhydrases

Protein-tyrosine-phosphatases (PTPases, EC 3.1.3.48) play a crucial role in the regulation of protein tyrosine phosphorylation. Recently, it was found that the PTPase gene family exhibits a large variety of different functional domains associated with the PTPase catalytic domains. In this paper, we report the complete cDNA sequence of a human transmembrane PTPase, PTPζ, isolated from fetal brain cDNA libraries. The deduced amino acid sequence of human PTPζ is composed of a putative signal peptide of 19 amino acids, a very large extracellular domain of 1616 amino acids, a transmembrane peptide of 26 amino acids, and a cytoplasmic domain of 653 amino acids. The extracellular portion of human PTPζ contains two striking structural features: the N-terminal 280-amino acid sequence that is homologous to carbonic anhydrases (carbonate hydro-lyase, EC 4.2.1.1), and a sequence of 1048 amino acids without a cysteine residue. While it is unlikely that the carbonic anhydrase-like domain of PTPζ has any carbonic anhydrase activity, its three-dimensional structure may be quite similar to that of carbonic anhydrases, a structure that appears ideal for binding a small soluble ligand. The cytoplasmic portion of human PTPζ contains two repeated PTPase-like domains, which, when expressed in Escherichia coli, had PTPase activity in vitro. Mutational analyses indicate that only the membrane-proximal PTPase domain is catalytically active. Reverse transcription-polymerase chain reaction analyses indicate that human PTPζ is highly expressed in a glioblastoma cell line.