Regulation of Ca2+/calmodulin-dependent protein kinase II by inter- and intrasubunit-catalyzed autophosphorylations.

Autophosphorylation of CaM kinase II on Thr286 is known to occur by an intraholoenzyme mechanism, but it is not known whether this reaction is intra- or intersubunit-catalyzed in the native heteromeric enzyme containing 10-12 alpha/beta subunits. In this study inactive CaM kinase II beta subunit, generated by mutation of Lys43 to Ala, and active kinase alpha subunit were expressed separately (homomeric kinases) or co-expressed (heteromeric kinase) using the baculovirus/Sf9 cell expression system and purified on CaM-Sepharose. Ca2+/CaM-dependent autophosphorylation of heteromeric alpha/beta kinase, which activated the enzyme, produced rapid autophosphorylation on Thr286 in both the active alpha and inactive beta subunits; the latter could only occur by intersubunit catalysis. Ca2+/CaM-independent autophosphorylation of nonactivated heteromeric kinase was slow, resulted in partial loss of total kinase activity, occurred only in the alpha subunit, and existed on Thr306 but not Thr286. This result demonstrates intrasubunit catalysis of Thr306 autophosphorylation. These observations that regulatory autophosphorylations of Thr286 and Thr306 were inter- and intrasubunit-catalyzed, respectively, have important consequences for structure/function models of CaM kinase II and for involvement of CaM kinase II autophosphorylation and activation during synaptic plasticity in neural systems.