Have you ever tried to unlock a door and had to try every key on the ring before finding the correct one? The mutational analysis of ion channels is sometimes similar. If you want to identify the structural determinant of a given channel property — gating, ion selectivity, ligand binding — you must mutate a lot of residues before obtaining a channel with the desired characteristics. And not only that; each amino acid that you mutate could in theory be replaced by 19 others, making it possible that the one replacement you tried was ineffective but this position was actually important for channel function. Two recent papers published in Neuron describe a yeast-based genetic screen to identify mutant channels more rapidly and on a larger scale than conventional approaches. Its use has already led to a deeper insight into the gating mechanism of G-protein-activated, inwardly rectifying K+ channels (GIRKs).

Yi et al. and Sadja et al. took advantage of a mutant yeast strain that lacks K+ transporters and therefore fails to grow in low K+ concentrations. But if a constitutively active K+ channel is expressed, then yeast can grow again. The authors of both papers subjected the entire sequence of two different GIRK channels to random mutagenesis, transformed mutant yeast with the mutated sequences, and looked at the colonies that managed to proliferate in low K+. As binding of βγG-protein subunits is required to open GIRK channels and the wild-type channel does not rescue the yeast phenotype, yeast growth in low K+ implied the expression of GIRK channels that became independent of G-protein activation. Further characterization of these G-protein-independent channels led the authors to identify several residues in the second transmembrane domain that were crucial for channel gating and to propose a structural mechanism for the transition between the closed and open states.

Remarkably, the residues identified in each paper were different despite the similarity of the approach. It would therefore be interesting to compare directly the rearrangements proposed in each article and test whether they are compatible with each other. But clearly, although this strategy allows you to test many more keys at a time, a lot of them seem to be capable of opening the gate.