A Missense Mutation in the Selectivity Filter of BK Affects the Channel's Potassium Conductance

Large-conductance Ca2+ activated potassium channels (BK), which are synergistically modulated by voltage and intracellular calcium, play important role in the control of the cell excitability. Here we report data from a de novo missense mutation in KCNMA1, encoding the BK channel α subunit found in a patient with progressive cerebellar ataxia. The mutation changes the residue of glycine 354, in the selectivity filter signature sequence 352-TVGYG-356 in potassium channels, into a serine. We expressed BK and G354S mutant BK (BK-G354S) channels in Xenopus oocytes and we analyzed macroscopic and on-cell single channel potassium currents recorded under voltage clamp. The macroscopic potassium currents are reduced in magnitude by about ten times and are slowly activated in oocytes expressing BK-G354S channels when compared with oocytes injected with cRNA for BK channels. The microscopic single channel conductance in BK-G354S is also depressed to about a tenth of the single channel conductance of BK channels, which suggest that the smaller macroscopic currents recorded with BK-G354S channels is not a mere problem of expression in the membrane. Actually, as revealed by expressing both proteins in mammalian cells and measuring their expression level using immunofluorescence techniques, BK-G354S channels are expressed in comparable levels with the expression of the wild-type BK channels. The mutation G354S in BK channel significantly decreases its potassium conductance, which importantly denotes a phenotype with loss of function. Biophysical data presented here provides crucial information that helps understanding the neurologic pathophysiology in the affected patient and it may also help in designing possible treatments for this and other similar cases. Supported by NIH GM030376, NS094665-01 and NS082788-02.