The type II calmodulin-dependent protein kinase is an oligomer existing in multiple isozymic forms. To facilitate investigations of the regulatory mechanisms of this complex enzyme, we have constructed a truncated, calmodulin-dependent monomer of the alpha subunit. The N-terminal enzyme fragment (alpha 315) was expressed at high levels in a baculovirus/insect cell expression system. The recombinant protein was purified chromatographically using DEAE-cellulose, calmodulin-Sepharose, and AffiGel blue, yielding 4 mg of kinase from 1.5 x 10(8) cells in 4 h. Characterization of the truncated kinase indicated that it is a monomer and that interactions of alpha 315 with calmodulin and substrates are indistinguishable from those observed for purified holoenzyme from rat brain. These results indicate that the baculovirus/insect cell expression system is well suited for producing alpha 315, a structurally simplified model of the type II calmodulin-dependent protein kinase.