Purification and characterization of the catalytic domains of the human receptor-linked protein tyrosine phosphatases HPTP beta, leukocyte common antigen (LCA), and leukocyte common antigen-related molecule (LAR)

Abstract

Human HPTP beta, leukocyte common antigen (LCA), and leukocyte common antigen-related molecule (LAR) are transmembrane receptor-like proteins whose cytoplasmic regions contain either one (HPTP beta) or two (LCA and LAR) domains that are homologous to protein tyrosine phosphatases (PTPases). Whereas the membrane-proximal domain 1 has enzymatic activity, the membrane-distal domain 2 of both LCA and LAR has no detectable catalytic activity. The cytoplasmic regions of HPTP beta, LCA, and LAR were expressed in Escherichia coli and purified to greater than 90% purity. Modulatory effects of various low molecular weight compounds and homo- and copolymers of amino acids were examined. Several polypeptides that contain a high proportion of tyrosine were strongly inhibitory to these PTPases. To determine a possible role for the LAR domain 2, the properties of recombinant LAR PTPases containing both domains 1 and 2 (LAR-D1D2) or only domain 1 (LAR-D1) were compared. In nearly all aspects examined, LAR-D1 and LAR-D1D2 were indistinguishable. However, polycationic polypeptides strongly stimulated the PTPase activity of LAR-D1D2, but not LAR-D1, using the peptide substrate Raytide. Thus, basic polypeptides seem to indirectly alter the catalytic activity of domain 1 by interacting with domain 2. This result suggests that domain 2 has a regulatory function.