Reconstitution of β-adrenergic modulation of large conductance, calcium-activated potassium (maxi-K) channels in Xenopus oocytes. Identification of the cAMP-dependent protein kinase phosphorylation site

The human large conductance, calcium-activated potassium (maxi-K) channel (α and β subunits) and β₂-adrenergic receptor genes were coexpressed in Xenopus oocytes in order to study the mechanism of β- adrenergic modulation of channel function. Isoproterenol and forskolin increased maxi-K potassium channel currents in voltage-clamped oocytes expressing the receptor and both channel subunits by 33 ± 5% and 35 ± 8%, respectively, without affecting current activation or inactivation. The percentage of stimulation by isoproterenol and forskolin was not different in oocytes coexpressing the α and β subunits versus those expressing the only the α subunit, suggesting that the α subunit is the target for regulation. The stimulatory effect of isoproterenol was almost completely blocked by intracellular injection of the cyclic AMP dependent protein kinase (cAMP-PK) regulatory subunit, whereas injection of a cyclic GMP dependent protein kinase inhibitory peptide had little effect, indicating that cellular coupling of β₂-adrenergic receptors to maxi-K channels involves endogenous cAMPK-PK. Mutation of one of several potential consensus cAMP-PK phosphorylation sites (serine 869) on the subunit almost completely inhibited β-adrenergic receptor/channel stimulatory coupling, whereas forskolin still stimulated currents moderately (16 ± 4%). These data demonstrate that physiological coupling between β₂ receptors and maxi-K channels occurs by the cAMP-PK mediated phosphorylation of serine 869 on the α subunit on the channel.