| Being able to be massively produced, purified, and crystallized, prokaryotic K+ channels have provided valuable insights in understanding the bases of ion filtration and gating that underlie all K+ channels in general. However, the K+ channel physiology of the microbes themselves is obscured. In an attempt to gain insight into the K+ channel functions, we focused on the established model organism, Escherichia coli. We randomly mutagenized the only K+-channel gene (kch) in its genome, and searched for mutations that cause cells to become sensitive to external K+. All the mutants isolated each has a single substitution mapped to the cytoplasmic regulatory domain of Kch, which presumably biases the channel towards its open conformation. A second mutation on the K+ filter of the mutant erases the K+ sensitivity caused by the first mutation, confirming that the K+-sensitive phenotype is due to a K+ leakage through the K+ filter of the mutant. Thus, we demonstrated that Kch makes a functional K+ conduit in vivo for the first time. Using the cellular ATP as an intrinsic indicator of proton motive force (PMF), we further showed that the mutant channel significantly reduces the PMF within thirty seconds when the external K+ concentration was suddenly increased. To gain additional functional insights of prokaryotic K+ channels in general, we also BLAST searched the NCBI database to identify and examine all of the possible K +-channel genes from sequenced 270 prokaryotic genomes. Based on the results of studying the E. coli Kch mutants, and of the genome-wide in silico analysis, we propose that: (1) Kch likely functions to adjust the energetic state of the E. coli membrane by regulating the membrane potential and proton motive force; (2) K + channels may also function to assist metabolic changes upon environmental changes in other prokaryotes; (3) Although widespread, K+ channels may not be essential to prokaryotes. |
