K⁺ CHANNELS PKC Helps BK Channels Choose

Abstract: The activation of large conductance Ca²⁺-activated K⁺ (BK) channels results in cellular hyperpolarization and decreased excitation. Protein kinase C (PKC), adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase (PKA), and guanosine 3',5'-monophosphate (cGMP)-dependent protein kinase (PKG) regulate the activity of BK channels. However, the mechanism by which these kinases regulate BK channel activity remains largely unknown. Zhou et al. cloned three isoforms of the BK channel subunit, which differ only in their COOH-termini: both the A (BK_A) and B (BK_B) isoforms have two serine residues that serve as sites for protein kinase C (PKC)-mediated phosphorylation, whereas the C (BK_C) isoform does not. Inside-out cell membrane patches from HEK293 cells that expressed BK_A or BK_B were activated by the application of PKG but not by PKA, whereas BK_C-containing inside-out patches were only activated by the application of PKA. Upon mutation of the PKC-mediated phosphorylation sites (serine to alanine) on BK_A, PKA (but not PKG) activated the mutated BK channel in inside-out patches, suggesting that PKC-dependent phosphorylation of BK_A switches the sensitivity of the BK channel from PKA to PKG. Mutation of either of the two PKC-mediated phosphorylation sites on BKA reduced, but did not completely eliminate, the capacity of PKG to activate BK_A, which suggests that both Ser residues in BK_A are important for optimal PKG-dependent activation of the BK channel. Mutation of the known distinct PKA or PKG phosphorylation sites on BK_A prevented the activation of BK channels by those kinases, indicating that PKA- or PKG-mediated phosphorylation is essential to BK channel activation. Thus, the data suggest that the activity of PKC determines the sensitivity of BK channel activation to PKA or PKG.

X.-B. Zhou, C. Arntz, S. Kamm, K. Motejlek, U. Sausbier, G.-X. Wang, P. Ruth, M. Korth, A molecular switch for specific stimulation of the BK_Ca channel by cGMP and cAMP kinase. J. Biol. Chem. 276, 43239-43245 (2001). [Abstract] [Full Text]