Conopeptide Vt3.1 preferentially inhibits BK potassium channels containing ß4 subunits via electrostatic interactions.

BK channel ß subunits (ß1-ß4) modulate the function of channels formed by slo1 subunits to produce tissue-specific phenotypes. The molecular mechanism of how the homologous ß subunits differentially alter BK channel functions and the role of different BK channel functions in various physiologic processes remain unclear. By studying channels expressed in Xenopus laevis oocytes, we show a novel disulfide-cross-linked dimer conopeptide, Vt3.1 that preferentially inhibits BK channels containing the ß4 subunit, which is most abundantly expressed in brain and important for neuronal functions. Vt3.1 inhibits the currents by a maximum of 71%, shifts the G-V relation by 45 mV approximately half-saturation concentrations, and alters both open and closed time of single channel activities, indicating that the toxin alters voltage dependence of the channel. Vt3.1 contains basic residues and inhibits voltage-dependent activation by electrostatic interactions with acidic residues in the extracellular loops of the slo1 and ß4 subunits. These results suggest a large interaction surface between the slo1 subunit of BK channels and the ß4 subunit, providing structural insight into the molecular interactions between slo1 and ß4 subunits. The results also suggest that Vt3.1 is an excellent tool for studying ß subunit modulation of BK channels and for understanding the physiological roles of BK channels in neurophysiology.

Interaction between residues in the Mg2+-binding site regulates BK channel activation.

As a unique member of the voltage-gated potassium channel family, a large conductance, voltage- and Ca(2+)-activated K(+) (BK) channel has a large cytosolic domain that serves as the Ca(2+) sensor, in addition to a membrane-spanning domain that contains the voltage-sensing (VSD) and pore-gate domains. The conformational changes of the cytosolic domain induced by Ca(2+) binding and the conformational changes of the VSD induced by membrane voltage changes trigger the opening of the pore-gate domain. Although some structural information of these individual functional domains is available, how the interactions among these domains, especially the noncovalent interactions, control the dynamic gating process of BK channels is still not clear. Previous studies discovered that intracellular Mg(2+) binds to an interdomain binding site consisting of D99 and N172 from the membrane-spanning domain and E374 and E399 from the cytosolic domain. The bound Mg(2+) at this narrow interdomain interface activates the BK channel through an electrostatic interaction with a positively charged residue in the VSD. In this study, we investigated the potential interdomain interactions between the Mg(2+)-coordination residues and their effects on channel gating. By introducing different charges to these residues, we discovered a native interdomain interaction between D99 and E374 that can affect BK channel activation. To understand the underlying mechanism of the interdomain interactions between the Mg(2+)-coordination residues, we introduced artificial electrostatic interactions between residues 172 and 399 from two different domains. We found that the interdomain interactions between these two positions not only alter the local conformations near the Mg(2+)-binding site but also change distant conformations including the pore-gate domain, thereby affecting the voltage- and Ca(2+)-dependent activation of the BK channel. These results illustrate the importance of interdomain interactions to the allosteric gating mechanisms of BK channels.

Ion sensing in the RCK1 domain of BK channels.

BK-type K(+) channels are activated by voltage and intracellular Ca(2+), which is important in modulating muscle contraction, neural transmission, and circadian pacemaker output. Previous studies suggest that the cytosolic domain of BK channels contains two different Ca(2+) binding sites, but the molecular composition of one of the sites is not completely known. Here we report, by systematic mutagenesis studies, the identification of E535 as part of this Ca(2+) binding site. This site is specific for binding to Ca(2+) but not Cd(2+). Experimental results and molecular modeling based on the X-ray crystallographic structures of the BK channel cytosolic domain suggest that the binding of Ca(2+) by the side chains of E535 and the previously identified D367 changes the conformation around the binding site and turns the side chain of M513 into a hydrophobic core, providing a basis to understand how Ca(2+) binding at this site opens the activation gate of the channel that is remotely located in the membrane.