Sequences of autophosphorylation sites in neuronal type II CaM kinase that control Ca²⁺-independent activity

After initial activation by Ca²⁺, the catalytic activity of type II Ca²⁺/calmodulin-dependent protein kinase rapidly becomes partially independent of Ca²⁺. The transition is caused by autophosphorylation of a few subunits in the dodecameric holoenzyme, which is composed of varying proportions of two homologous types of subunits, α (50 kd) and β (58-60 kd). We have identified one site in the α subunit (Thr₂₈₆) and two in the β subunit (Thr₂₈₇ and Thr₃₈₂) that are rapidly autophosphorylated. We show that phosphorylation of α-Thr₂₈₆ and β;-Thr₂₈₇, which are located immediately adjacent to the calmodulin binding domain, controls Ca²⁺-independent activity. In contrast, phosphorylation of β-Thr₃₈₂ is not required to maintain Ca²⁺ independence. It is absent in the a subunit and is selectively removed from the minor β subunit, apparently by alternative splicing. Regulation of the presence of β-Thr₃₈₂ in the holoenzyme by both differential gene expression and alternative splicing suggests that it may have an important but highly specialized function.