Role of the S4 in cooperativity of voltage-dependent potassium channel activation

Charged residues in the S4 transmembrane segment of voltage-gated cation channels play a key role in opening channels in response to changes in voltage across the cell membrane. However, the molecular mechanism of channel activation is not well understood. To learn more about the role of the S4 in channel gating, we constructed chimeras in which S4 segments from several divergent potassium channels, Shab, Shal,:Shaw, and k(v)3.2, were inserted into a Shaker potassium channel background. These S4 donor channels have distinctly different voltage-dependent gating properties and S4 amino acid sequences. None of the S4 chimeras have the gating behavior of their respective S4 donor channels. The conductance-voltage relations of all S4 chimeras are shifted to more positive voltages and the slopes are decreased. There is no consistent correlation between the nominal charge content of the S4 and the slope of the conductance-voltage relation, suggesting that the mutations introduced by the S4 chimeras may alter cooperative interactions in the gating process. We compared the gating behavior of the Shaw S4 chimera with its parent channels, Shaker and Shaw, in detail. The Shaw S4 substitution alters activation gating profoundly without introducing obvious changes in other channel functions. Analysis of the voltage-dependent gating kinetics suggests that the dominant effect of the Shaw S4 substitution is to alter a single cooperative transition late in the activation pathway, making it rate limiting. This interpretation is supported further by studies of channels assembled from tandem heterodimer constructs with both Shaker and Shaw S4 subunits. Activation gating in the heterodimer channels can be predicted from the properties of the homotetrameric channels only if it is assumed that the mutations alter a cooperative transition in the activation pathway rather than independent transitions.