CALPAIN-CATALYZED CLEAVAGE AND SUBCELLULAR RELOCATION OF PROTEIN PHOSPHOTYROSINE PHOSPHATASE-1B (PTP-1B) IN HUMAN PLATELETS

The non-transmembrane phosphotyrosine phosphatase 1B (PTP-1B) is an ab undant enzyme, normally localized to the cytosolic face of the endopla smic reticulum via a C-terminal targeting sequence. We have found that agonist-induced platelet activation results in proteolytic cleavage o f PTP-1B at a site upstream from this targeting sequence, causing subc ellular relocation of its catalytic domain from membranes to the cytos ol. PTP-1B cleavage is catalyzed by the calcium-dependent neutral prot ease calpain and is a general feature of platelet agonist-induced aggr egation. Moreover, PTP-1B cleavage correlates with the transition from reversible to irreversible platelet aggregation in platelet-rich plas ma. Engagement of gpIIb-IIIa is necessary for inducing PTP-1B cleavage , suggesting that integrins regulate tyrosine phosphatases as well as tyrosine kinases. PTP-1B cleavage is accompanied by a 2-fold stimulati on of its enzymatic activity, as measured by immune complex phosphatas e assay, and correlates with discrete changes in the pattern of tyrosy l phosphorylation. Cleavage and subcellular relocation of PTP-1B repre sents a novel mechanism for altering tyrosyl phosphorylation that may have important physiological implications in cell types other than pla telets.