RCK Domain Model of Calcium Activation in BK Channels

Abstract

Potassium ion channels are ubiquitously expressed from bacteria to mammals where they are involved in various processes ranging from the regulation of osmotic pressure in a single cell to the electrical response in muscle fibers to the generation of action potentials in neurons. The B K channel family (BK for Big K+ conductance) is an interesting subfamily of K+ channels responsive to both membrane voltage and intracellular calcium ion. The unique, high-affinity Ca2+ sensitivity of B K channels is critical to their physiological function in various cell types. The mechanism by which Ca2+ activates B K channel gating, however, is not well understood. Here we present a structure-based approach to the study of B K channels with the goal of providing a structural and functional model of the Ca2+-activation mechanism. Sequence analysis of BK channel C-terminal domains and domains from prokaryotic homologs reveals the conservation of unique positions defining a novel regulatory domain associated with K conduction, the R C K domain. Crystal structures of R C K domains from prokaryotic sources relate the conservation of sequence to the structure, assembly and function of these domains. W e propose a hypothetical model for the structure and function of the Cterminal domains of B K as a set of R C K domains that conduct the Ca -activation mechanism. The features and constraints predicted by the R C K domain model are tested by the electrophysiological assay of a variety of human B K constructs. The results support a domain structure and assembly consistent with the proposed model for the B K C-terminus. In addition, the results identify residues and regions involved in Ca + activation: the Ca2+-binding event and the transduction of the binding energy through protein conformational changes to the channel domain. The R C K domain model thus provides a framework for the study of Ca2+ activation in B K channels.