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Homology model of heteromeric Kir4.1/Kir5.1. For clarity, only TM2 and the pore α-helices are shown. Kir4.1 TM2 is shown in yellow, Kir5.1 TM2 in light blue. The upper hinge residues (Kir4.1-T154, Kir5.1-S157) and the putative lower hinge glycines (Kir4.1-G163, Kir5.1-G166) are indicated as vdwspheres in cpk. Enlarged views of the upper hinge area of Kir4.1 showing T154 and of the lower hinge areas are shown on the right hand side
This model shows that instead of being isolated within TM2, both
“upper hinge” residues (T154 in Kir4.1 and S157 in Kir5.1) are in
direct contact with residues at the base of the pore α-helix, which
precedes the selectivity filter (Fig. 6 ). .
Shang, Lijun; Tucker, Stephen J.Journal: European Biophysics Journal
Issue 2DOI: 10.1007/s00249-007-0206-7Published: 2008-01-09Institution(s):
University of Oxford
Comparison of the crystal structures of the KcsA and MthK potassium channels suggests that the process of opening a K+ channel involves pivoted bending of the inner pore-lining helices at a highly conserved glycine residue. This bending motion is proposed to splay the transmembrane domains outwards to widen the gate at the “helix-bundle crossing”. However, in the inwardly rectifying (Kir) potassium channel family, the role of this “hinge” residue in the second transmembrane domain (TM2) and that of another putative glycine gating hinge at the base of TM2 remain controversial. We investigated the role of these two positions in heteromeric Kir4.1/Kir5.1 channels, which are unique amongst Kir channels in that both subunits lack a conserved glycine at the upper hinge position. Contrary to the effect seen in other channels, increasing the potential flexibility of TM2 by glycine substitutions at the upper hinge position decreases channel opening. Furthermore, the contribution of the Kir4.1 subunit to this process is dominant compared to Kir5.1, demonstrating a non-equivalent contribution of these two subunits to the gating process. A homology model of heteromeric Kir4.1/Kir5.1 shows that these upper “hinge” residues are in close contact with the base of the pore α-helix that supports the selectivity filter. Our results also indicate that the highly conserved glycine at the “lower” gating hinge position is required for tight packing of the TM2 helices at the helix-bundle crossing, rather than acting as a hinge residue.
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