Allosteric interactions and the modular nature of the voltage- and Ca2+-activated (BK) channel

The high conductance voltage- and Ca2+-activated K+ channel is one of the most broadly expressed channels in mammals. This channel is named BK for ‘big K’ because of its single-channel conductance that can be as large as 250 pS in 100 mm symmetrical K+. BK channels increase their activity by membrane depolarization or an increase in cytosolic Ca2+. One of the key features that defines the behaviour of BK channels is that neither Ca2+ nor voltage is strictly necessary for channel activation. This and several other observations led to the idea that both Ca2+ and voltage increase the open probability by an allosteric mechanism. In this type of mechanism, the processes of voltage sensor displacement, Ca2+ binding and pore opening are independent equilibria that interact allosterically with each other. These allosteric interactions in BK channels reside in the structural characteristics of the BK channel in the sense that voltage and Ca2+ sensors and the pore need to be contained in different structures or ‘modules’. Through electrophysiological, mutagenesis, biochemical and fluorescence studies these modules have been identified and, more important, some of the interactions between them have been unveiled. In this review, we have covered the main advances achieved during the last few years in the elucidation of the structure of the BK channel and how this is related with its function as an allosteric protein

Modulation of spike clustering by NMDA receptors and neurotensin in rat supraoptic nucleus neurons

Photograph of a scene at the Dinosaur Tracks area, along the Santa Fe Trail

ΔN-TRPV1: A Molecular Co-detector of Body Temperature and Osmotic Stress

Thirst and antidiuretic hormone secretion occur during hyperthermia or hypertonicity to preserve body hydration. These vital responses are triggered when hypothalamic osmoregulatory neurons become depolarized by ion channels encoded by an unknown product of the transient receptor potential vanilloid-1 gene (Trpv1). Here, we show that rodent osmoregulatory neurons express a transcript of Trpv1 that mediates the selective translation of a TRPV1 variant that lacks a significant portion of the channel’s amino terminus (ΔN-TRPV1). The mRNA transcript encoding this variant (Trpv1dn) is widely expressed in the brains of osmoregulating vertebrates, including the human hypothalamus. Transfection of Trpv1dn into heterologous cells induced the expression of ion channels that could be activated by either hypertonicity or by heating in the physiological range. Moreover, expression of Trpv1dn rescued the osmosensory and thermosensory responses of single hypothalamic neurons obtained from Trpv1 knockout mice. ΔN-TRPV1 is therefore a co-detector of core body temperature and fluid tonicity

Role of the electrostatic potential on the BK potassium channel conductance

Fernando D. Gonzalez-Nilo and Wendy Gonzalez. Centro de Bioinformatica, Universidad de Talca, Talca, Chile.The family of K channels presents a highly conserved structural motif (TVGYG), known as the selectivity filter. Despite the fact that the selectivity filter is conserved in all K channels, they show different conductances. We observed that the number of negative charged aminoacids in the outer vestibule of BK (high conductance), KvAP (medium conductance) and Shaker (low conductance) K channels, is proportional to the conductance of each channel. In particular, BK channel has a loop bearing two charged aminoacids in the outer vestibule. These negative charges would increase the local K ion concentration in the outer vestibule of the pore, diminishing the required energy to access the selectivity filter. To verify experimentally this hypothesis a series of BK channel mutants were constructed and the single channel conductances were measured. In agreement with our predictions, the removal of the loop charges (D261N/E264Q) leads to a reduced inward conductance. Deleting the outer loop does not affect inward conductance unless another charge is removed (Δloop/E276Q). This suggests that E276 can compensate the electrostatic contribution of the loop charges when the loop is deleted. The results are in agreement with electrostatic potential calculations (Poisson-Boltzmann) carried out on a molecular model of the BK channel pore. In conclusion, we have identified residues involved in the electrostatic tuning of the BK channel inward conductance. Acknowledgment: FONDECYT 1040254 (FG) and 1030830 (RL). Fundación Andes (FG)Meeting Abstract, Presentation Number: 483-Pos Poster Board Number: B322