Ion channels

Abstract:

Inwardly rectifying K+ channel subunits may form homomeric or heteromeric channels with distinct functional properties. Hyperpolarizing commands delivered to Xenopus oocytes expressing homomeric Kir 4.1 channels evoke inwardly rectifying K+ currents which activate rapidly and undergo a pronounced decay at more hyperpolarized potentials. In addition, Kir 4.1 subunits form heteromeric channels when coexpressed with several other inward rectifier subunits. However, coexpression of Kir 4.1 with Kir 3.4 causes an inhibition of the Kir 4.1 current. We have investigated this inhibitory effect and show that it is mediated by interactions between the predicted transmembrane domains of the two subunit classes. Other subunits within the Kir 3.0 family also exhibit this inhibitory effect which can be used to define subgroups of the inward rectifier family. Further, the mechanism of inhibition is likely due to the formation of an "inviable complex" which becomes degraded, rather than by formation of stable nonconductive heteromeric channels. These results provide insight into the assembly and regulation of inwardly rectifying K+ channels and the domains which define their interactions.

Abstract:

Coexpression in Xenopus oocytes of the cloned cardiac inward rectifier subunits K(ir) 3.1 and K(ir) 3,4 results in G protein-stimulated channel activity closely resembling the muscarinic channel underlying the inwardly rectifying K+ current in atrial myocytes. To determine the stoichiometry and relative subunit positions within the channel, K(ir). 3.1 and K(ir) 3.4 were eoexpressed in varying ratios with cloned Gβ1γ2 subunits and also as tandemly linked tetramers with different relative subunit positions. The results reveal that the most efficient channel comprises two subunits of each type in an alternating array within the tetramer. To localize regions important for subunit coassembly and G protein sensitivity, chimeric subunits containing domains from either K(ir) 3.1, K(ir) 3.4, or the G protein- insensitive subunit K(ir) 4.1 were expressed. The results demonstrate that the transmembrane domains dictate the potentiation of the coassembled channels and that, although the NH4- or COOH-termini of both subunits alone can confer G protein sensitivity, both termini are required for maximal stimulation by Gβ1γ2.