Selective dephosphorylation of the threonine183 residue of ERK2 upon αllbβ3 engagement in platelets

Thrombin‐induced extracellular signal‐regulated kinase 2 (ERK2) activation is negatively regulated in conditions of αllbβ3 integrin engagement and platelet aggregation. Here we show by Western blotting with antibodies against mono‐ and biphosphorylated forms of ERK2 that the dephosphorylation of ERK2 by αllbβ3 engagement affects threonine 183 and not tyrosine 185 . Addition of a potent serine/threonine phosphatase inhibitor, okadaic acid (OA), restored thrombin‐induced threonine phosphorylation of ERK2 in conditions of platelet aggregation, whereas OA had no effect in the absence of αllbβ3 engagement. These observations are consistent with αllbβ3 engagement acting via at least one serine/threonine phosphatase, which dephosphorylates the phosphothreonine 183 residue of ERK2. Moreover, a small amount (14%) of ERK2 was translocated to the αllbβ3‐dependent cytoskeleton, mostly in a monophosphorylated (i.e. inactive) form, suggesting that cytoskeleton‐associated ERK2 plays only a minor role, if any. Finally, we show that negative regulation (i.e. dephosphorylation) occurs primarily or totally in the cytosol and that the αllbβ3‐dependent ERK2 Thr 183 ‐specific phosphatase is different from phosphatase 1 (PP1) or PP2A. We conclude that αllbβ3 engagement down‐regulates ERK2 through selective dephosphorylation of the phosphothreonine 183 residue by a cytosolic serine/threonine phosphatase different from known platelet phosphatases.