Voltage-gated K+ channels isolated from mammalian brain are composed of α and β subunits. Interaction between coexpressed K(v)1.1 (α) and K(v)β1.1 (β) subunits confers rapid inactivation on the delayed rectifier- type current that is observed when α subunits are expressed alone. Integrating electrophysiological and biochemical analyses, we show that the inactivation of the αβ current is not complete even when α is saturated with β, and the αβ current has an inherent sustained component, indistinguishable from a pure α current. We further show that basal and protein kinase A-induced phosphorylations at Ser-446 of the α protein increase the extent, but not the rate, of inactivation of the αβ channel, without affecting the association between α and β. In addition, the extent of inactivation is increased by agents that lead to microfilament depolymerization. The effects of phosphorylation and of microfilament depolymerization are not additive. Taken together, we suggest that phosphorylation, via a mechanism that involves the interaction of the αβ channel with microfilaments, enhances the extent of inactivation of the channel. Furthermore, phosphorylation at Ser-446 also increases current amplitudes of the αβ channel as was shown before for the α channel. Thus, phosphorylation enhances in concert inactivation and current amplitudes, thereby leading to a substantial increase in A-type activity.