FIGURE 8.

Physical location of Subregion B within RCK1. Alignment of the amino acid sequences of the RCK1 domains of both eukaryotic (SLO3 and SLO1) and prokaryotic potassium channels (MethK and A. aeo2TM) based on the work of Jiang et al. (21) show the position of subregion B. Because the sequence of SLO3 potassium channels is co-linear and conserved with that of SLO1 potassium channels, we were able to add the SLO3 RCK1 domain to that alignment. This alignment revealed that subregion B is a loop between the αG helix and the βG strand of the RCK1 domain and serves to link the αG helix of the intermediate subdomain to the C-terminal subdomain. This loop region contains the greatest sequence and length polymorphisms among RCK1 domains of many species, as shown by the RCK1 sequence alignment of Ref. 21. This region also has unusually low sequence conservation in bovine and mouse SLO3 channels (see Fig. 1A, as well as this figure). The arrow in the three-dimensional molecular diagram indicates the loop position as based on the crystal structure (21). The presence of RCK domains in SLO1 channels was first reported by Jiang et al. (21), who inferred their presence in SLO1 channels by their sequence homology to the RCK domains of MthK and other prokaryotic ion channels. The presence of RCK domains in SLO3 channels is inferred by the high amino acid sequence homology of the mSLO1 and mSLO3 paralogues (4). Notably, the sequence identity between the putative RCK1 domains of SLO1 and SLO3 channels is much higher (>50%) than the sequence identity between putative RCK1 domains of SLO1 and MthK channels (<20%), from which the presence of RCK domains in SLO1 family channels was originally inferred. A published study has also concluded that both RCK1 and RCK2 domains are present in all SLO family channels, SLO1, SLO2, and SLO3 (30).