Regulation of protein-tyrosine phosphatases alpha and epsilon by calpain-mediated proteolytic cleavage

The precise subcellular localization of non-receptor tyrosine phosphatases is a major factor in regulating their physiological functions. We have prev iously shown that cellular processing of protein-tyrosine phosphatase epsil on (PTP epsilon) generates a physiologically distinct, cytoplasmic form of this protein, p65 PTP epsilon. Here we describe a novel protein form of the related receptor-type tyrosine phosphatase alpha (RPTP alpha), p66 PTP alp ha, which is detected in nearly all cell types where RPTPa is expressed. Bo th p66 PTP alpha and p65 PTP epsilon are produced by calpain-mediated prote olytic cleavage in vivo. Cleavage is inhibited in living cells by a variety of calpain inhibitors, can be induced in primary cortical neurons treated with calcium chloride, and is observed in lysates of brain or of cultured c ells following addition of purified calpain. Cleavage occurs within the int racellular juxtamembrane domain of RPTPa, releasing the phosphatase catalyt ic domains from their membranal anchors and translocating them to the cytop lasm. Translocation reduces the ability of PTPa to act on membrane-associat ed substrates, as it loses its ability to dephosphorylate Src at its C-term inal regulatory site, and its ability to dephosphorylate the Kv2.1 voltage- gated potassium channel is severely impaired. In all, the data indicate tha t control of phosphatase function via post-translational processing occurs also among receptor-type phosphatases, and demonstrate the molecular comple xity of regulating these parameters within the PTP alpha /PTP epsilon phosp hatase subfamily.