Protein kinase A phosphorylation alters Kvβ1.3 subunit-mediated inactivation of the Kv1.5 potassium channel

The human Kv1.5 potassium channel forms the I-Kur current in atrial myocytes and is functionally altered by coexpression with Kv beta subunits. To explore the role of protein kinase A (PKA) phosphorylation in beta-subunit function, we examined the effect of PKA stimulation on Kv1.5 current following coexpression with either Kv beta 1.2 or Kv beta 1.3, both of which coassemble with Kv1.5 and induce fast inactivation. In Xenopus oocytes expressing Kv1.5 and Kv beta 1.3, activation of PKA reduced macroscopic inactivation with an increase in K+ current. Similar results were obtained using HEK 293 cells which lack endogenous K+ channel subunits. These effects did not occur whom Kv1.5 was coexpressed with either KV beta 1.2 or Kv beta 1.3 lacking the amino terminus,suggesting involvement of this region of Kv beta 1.3. Removal of a consensus PKA phosphorylation site on the Kv beta 1.3 NH2 terminus (serine 24), but not alternative sites in either Kv beta 1.3 or Kv1.5, resulted in loss of the functional effects of kinase activation. The effects of phosphorylation appeared to be electrostatic, as replacement of serine 24 with a negatively charged amino acid reduced beta-mediated inactivation, while substitution with a positively charged residue enhanced it. These results indicate that Kv beta 1.3-induced inactivation is reduced by PKA activation, and that phosphorylation of serine 24 in the subunit NH2 terminus is responsible.