Modulation of voltage sensitivity by N-terminal cytoplasmic residues in human Kv1.2 channels

Potassium channels are now among the best understood membrane proteins and most salient functions have been mapped onto distinct portions of the protein. The detailed mechanism by which movement of the voltage sensor is transduced into channel opening is yet to be understood. We have constructed chimaeras from our collection of human voltage-gated potassium channels and expressed them in Xenopus oocytes. Here we report on a chimaeric construct, 1N/2, generated by swapping the N-terminal cytoplasmic residues of hKv1.1 onto the transmembrane body of hKv1.2. This chimaera functions as a classic outward rectifier but with a 25 mV hyperpolarizing shift in the mid-point of channel activation. The conductance of oocytes expressing this construct decreases significantly on depolarizing beyond +5 mV, unlike full-length hKv1.2. Other parameters such as ionic selectivity and charybdotoxin blockage are unaffected in making the chimaera. These observations suggest that the introduction of the "foreign" chain from hKv1.1 does not cause a large-scale perturbation of channel structure. Loss of the N-terminus from hKv1.2 is not responsible for the shift in voltage dependence, as a truncation construct, delta75N2, starting at the splice junction, has the same voltage-dependence as full-length hKv1.2. Both constructs show a maximum in their conductance-voltage curves. This decline in conductance on extensive depolarization may arise due to perturbations to the machinery that locks channels into their open state on depolarization. Taken together with our observations on other N-terminal swapped chimaeras, our data imply that N-terminal residues can interact with transmembrane regions and perturb the machinery mediating voltage-dependent channel gating.