Intrinsic Gating in Inward Rectifier Potassium Channels (Kir2.1) with Low Polyamine Affinity Generated by Site Directed Mutagenesis

Intrinsic Gating in Inward Rectifier Potassium Channels (Kir2.1) with Low Polyamine Affinity Generated by Site Directed Mutagenesis

We have studied mutant forms of Kir2.1 in which an aspartate residue (D172), important for gating by intracellular polyamines, is replaced by one of three basic residues (Arg, Lys or His). Such channels are highly selective for K<SUP>+</SUP>, but show inward rectification that is a shallow function of voltage compared with that found in wild type. This inward rectification occurs with a reduced affinity for spermine and persists in the absence of polyamines. Though the unitary current-voltage relation shows some inward rectification, it is insufficient to account for that seen under whole cell recording. Channels open and shut under single channel recording, and changes of <I>P</I><SUB>open</SUB> appear to generate inward rectification. In D172H, the reduction in affinity for spermine is greater when His is protonated at low pH<SUB>i</SUB>. The effective valency for spermine is reduced from 3.09&#8273;0.07 in wild type to 1.95&#8273;0.09 in D172H at pH<SUB>i</SUB> 6.3. In the presence of dual mutants of Kir2.1, where E224 is also replaced, spermine affinity becomes undetectable. However, channels still show inward rectification and open and shut under hyper- and depolarisation, respectively. We suggest that Kir2.1 channel are able to undergo conformation changes; these changes may be important physiologically in generating inward rectification, the normal parameters of which are set by the binding of polyamines such as spermine.