Observation of σ-pore currents in mutant hKv1.2_V370C potassium channels

Abstract

Current through the σ-pore was first detected in hKv1.3_V388C channels, where the V388C mutation in hKv1.3 channels opened a new pathway (σ-pore) behind the central α-pore. Typical for this mutant channel was inward current at potentials more negative than -100 mV when the central α-pore was closed. The α-pore blockers such as TEA+ and peptide toxins (CTX, MTX) could not reduce current through the σ-pore of hKv1.3_V388C channels. This new pathway would proceed in parallel to the α-pore in the S6-S6 interface gap. To see whether this phenomenon is restricted to hKv1.3 channels we mutated hKv1.2 at the homologue position (hKv1.2_V370C). By overexpression of hKv1.2_V370C mutant channels in COS-7 cells we could show typical σ-currents. The electrophysiological properties of the σ-pore in hKv1.3_V388C and hKv1.2_V370C mutant channels were similar. The σ-pore of hKv1.2_V370C channels was most permeable to Na+ and Li+ whereas Cl- and protons did not influence current through the σ-pore. Tetraethylammonium (TEA+), charybdotoxin (CTX) and maurotoxin (MTX), known α-pore blockers, could not reduce current through the σ-pore of hKv1.2_V370C channels. Taken together we conclude that the observation of σ-pore currents is not restricted to Kv1.3 potassium channels but can also be observed in a closely related potassium channel. This finding could have implications in the treatment of different ion channel diseases linked to mutations of the respective channels in regions close to homologue position investigated by us.

Characterization of the σ-Pore in Mutant hKv1.3 Potassium Channels

“…Together with the observation of the σ-pore in a closely related potassium channel ( h K v 1.2_V370C) [11] we conclude that the σ-pore may be a structural element common in a variety of voltage-gated ion channels. This in turn, might have implications for the treatment of those ion channel diseases that are associated with mutations in the pore region [14].…”

“…1 B ) with an inward current dip at ∼ -40 mV indicating current flow through the central potassium-selective α-pore. To measure more precisely the kinetic behavior of the current through the α- and σ-pore simultaneously we performed measurements using voltage steps similar to those described before [10, 11] and shown in Fig. 1 D - F .…”

“…A different ion conducting pathway, the σ-pore, was described in mutant voltage-gated potassium channels in which a valine was replaced by a cysteine at position 388 in h K v 1.3 channels (Shaker position 438) or at the homologue position 370 in h K v 1.2 channels [10, 11]. This mutation showed, in addition to the current through the central, potassium-selective α-pore, an additional inward current at membrane potentials more negative than -100 mV, i.e.…”

“…at potentials where the α-pore is normally closed. The inward current of the mutant channel was similar but not identical to the ω-current in the VSD-mutant channels, for example, Na + and Li + could carry large inward σ-current whereas ions like K + and Rb + only generated small inward σ-currents [10, 11]. In addition, current through the σ-pore was blocked by verapamil and not influenced by CTX, KTX or MTX [10, 11].…”

“…The inward current of the mutant channel was similar but not identical to the ω-current in the VSD-mutant channels, for example, Na + and Li + could carry large inward σ-current whereas ions like K + and Rb + only generated small inward σ-currents [10, 11]. In addition, current through the σ-pore was blocked by verapamil and not influenced by CTX, KTX or MTX [10, 11]. According to the model of the mutant h Kv1.3_V388C channel [10], the σ-pore was located behind the central α-pore at the back of the selectivity filter and proceeded in parallel to the central α-pore.…”