BK Channel News

The large conductance Ca 2 -activated K channel (BK Ca ) is well positioned to assume diverse physiological functions because it is ubiquitously expressed in many cell types. Some of the functions recently proposed for this channel offer unexpected surprises, however, such as a role in innate immunity in neutrophil leukocytes (Ahluwalia et al., 2004), recognition as a heme-binding protein (Tang et al., 2003), behavioral responses to ethanol (Davies et al., 2003), and a protective mechanism against ischemic cell death in the inner mitochondrial membrane of cardiac myocytes (Xu et al., 2002). These novel occupations attributed to BK Ca remain to be fully explored and validated before placing them along side the well-established physiological functions of this channel in the negative feedback regulation of voltagegated Ca 2 entry into nerve terminals (Robitaille et al., 1993), smooth muscle (Brayden and Nelson, 1992), and auditory hair cells of birds and turtles (Duncan and Fuchs, 2003). Nonetheless, the more we learn about the diverse roles of BK Ca in the lives of cells, the greater our incentive to understand exactly how this hyperpolarizing K -channel protein simultaneously senses both membrane voltage and intracellular Ca 2 . Particularly so because BK Ca is a member of a multigene family that includes Slo3, a Ca 2 -insensitive K channel predominantly expressed in spermatocytes (Schreiber et al., 1998), and Slack and Slick, two closely related genes that encode Na -activated K channels (Bhattacharjee et al., 2003; Yuan et al., 2003). The surge in attention that BK Ca has attracted is fueled partly by the fact that cloned BK Ca genes (e.g., Drosophila slowpoke ) are readily expressed in oocytes and cultured cell lines and that single-channel currents of BK Ca can be recorded at high resolution over a wide range of open state probability. Biophysical analysis of single-channel, macroscopic, and gating currents previously led to the development of a 50-state kinetic model describing two-tiered allosteric activation of a voltage-dependent, homotetrameric channel protein that binds one Ca 2 ion per subunit in both closed and open states (Cox and Aldrich, 2000; Rothberg and Magleby, 2000). However, the most recent studies suggest that there are at least three Ca 2 -activation sites per subunit, two high-affinity, Ca 2 -specific sites and one low-affinity site that binds both Ca 2 and Mg 2 (Zhang et al., 2001; Bao et al., 2002; Shi et al., 2002; Xia et al., 2002). A recent review of the BK Ca gating mechanism in this journal (Magleby, 2003) placed the latest tally of the “minimal” number of kinetic states at either 1,250 or 1,875, depending on whether we allow the luxury of a second tier of closed states. We are a long way afield from early modeling efforts, which attempted to limit the number of kinetic intermediates spanning the closed and open states to a meager handful (Methfessel and Boheim, 1982; Moczydlowski and Latorre, 1983). The modern kinetic analysis has provided powerful quantitative models capable of simulating the gating behavior of BK Ca with high fidelity. The next challenge is the identification of the molecular basis for the different states, and several research groups have accepted this challenge by seeking biochemical evidence that may reveal structural information on the nature of the divalent cation binding sites and conformational changes that underlie the 1,875 intermediates of BK Ca gating. This issue of the Journal of General Physiology treats us to a fresh perspective on this endeavor in the article by Bao et al. (2004). This article focuses on a linear sequence of 28 residues, Thr883-Gln910, near the intracellular COOH-terminal end of mSlo, the channel-forming subunit encoded by the mouse BK Ca gene. The functional significance of this particular region has been a subject of intense interest since Schreiber and Salkoff (1997) first named it the “Ca 2 bowl” and characterized it as a highly conserved motif in BK Ca that is rich in acidic residues (eight Asp and two Glu residues) and results in greatly diminished Ca 2 -sensitivity when altered by various mutations. Remarkably, Schreiber et al. (1999) also showed that a spliced segment containing the Ca 2