Journal of Biological Chemistry
Volume 270, Issue 34, 25 August 1995, Pages 20067-20072
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Enzymology
Biochemical Characterization of a Human Band 4.1-related Protein-tyrosine Phosphatase, PTPH1 (∗)

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PTPH1 is a human protein-tyrosine phosphatase with homology to the band 4.1 superfamily of cytoskeleton-associated proteins. Here, we report the purification and biochemical characterization of this enzyme from baculovirus-infected insect cells. The purified protein exhibited an apparent Mr of 120,000 on SDS gels. The native enzyme dephosphorylated both myelin basic protein (MBP) and reduced, carboxamidomethylated, and maleylated lysozyme (RCML) but was over 5-fold more active on MBP. The Km values for the two substrates were similar (1.45 μM for MBP and 1.6 μM for RCML). Phosphorylation of PTPH1 by protein kinase C in vitro resulted in a decrease in Km but had no effect on Vmax. Removal of the NH2-terminal band 4.1 homology domain of PTPH1 by limited trypsin cleavage stimulated dephosphorylation of RCML but inhibited its activity toward MBP. The dephosphorylation of RCML by full-length PTPH1 was enhanced up to 6-fold by unphosphorylated MBP and increasing ionic strength up to 0.2 M NaCl, whereas trypsinized preparations of PTPH1 containing the isolated catalytic domain were unaffected. These results suggest that in addition to a potential role in controlling subcellular localization, the NH2-terminal band 4.1 homology domain of PTPH1 may exert a direct effect on catalytic function.

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This work was supported by Grant CA53840 from the National Cancer Institute. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

§

Cold Spring Harbor Association Fellow.

Supported by the Council for Tobacco Research, U.S.A. Inc. Present address: Dept. of Biology, Howard University, Washington, D. C. 20059.

∗∗

Present address: Gene Therapy Center, 944 Wilson Hall CB#7352, University of North Carolina, Chapel Hill, NC 27599.