Structure Watch

Open the gate...

© (2002) Macmillan Publishers Ltd

Ion channels open and close in response to a stimulus that 'gates' the channel. But how does gating occur, and how do pores open? In Nature, new insights have now been provided by two papers from the MacKinnon group.

In the first paper, MacKinnon's group presents the structural basis of ligand gating in a K⁺ channel that opens in response to intracellular Ca²⁺. They determined the 3.3-Å resolution crystal structure of MthK from Methanobacterium thermoautotrophicum in its Ca²⁺-bound, open conformation. The channel is tetrameric, and the subunits that form the pore (top of figure) are each made up of two transmembrane segments. Each subunit also has a 'regulator of K⁺ conductance' (RCK) domain at the intracellular surface, although MthK actually has eight RCK domains (bottom of figure), as four RCK domains join the complex from the intracellular solution.

The RCK domains form a 'gating ring' through a pattern of alternate 'fixed' and 'flexible' interfaces, which actually makes four rigid units (RCK-domain dimers joined by the fixed interface). The flexible interfaces form ligand-binding clefts between RCK domains, and two Ca²⁺ ions (yellow circles) ? which are directly correlated with channel gating ? bind to each of these clefts.

By comparing the structure of the Ca²⁺-bound RCK domain of MthK with that of an unbound RCK domain from an Escherichia coli K⁺ channel, the authors gained insight into how the gating ring converts the free energy of Ca²⁺ binding into mechanical changes in the pore. They propose that Ca²⁺ binding to the cleft reshapes it, so that the rigid units tilt and expand the diameter of the gating ring. This, in turn, pulls open the inner helices of the pore (see dashed lines) and lets ions pass through.

...and the pore

In the second paper, the group investigated how a pore opens by comparing the 'open' MthK structure with the known 'closed' structure of KcsA ? a K⁺ channel from Streptomyces lividans. Although the region around the ion selectivity filter is similar in both channels, the authors noticed large structural differences in the inner helices of the two pores.

The helices are almost straight in KcsA, and form a bundle that closes the pore near its intracellular opening. However, in MthK, the helices are bent and splayed open, which produces a wide pore. The bending point corresponds to a glycine ? the most flexible amino acid ? that is located deep inside the membrane, and MacKinnon's group found that this 'hinge' residue is conserved in a wide range of K⁺ channels.

These observations fit neatly with the gating mechanism described above ? ligand binding reorganizes the gating ring, which exerts a radial force that focuses at the hinge and causes the inner helices of the pore to bend outwards, thus opening the pore. REFERENCES Jiang, Y. et al. Crystal structure and mechanism of a calcium-gated potassium channel. Nature 417, 515?522 (2002) Jiang, Y. et al. The open pore conformation of potassium channels. Nature 417, 523?526 (2002)