fig4: Normalized QON-V relationships from Kv4.2CTX and Kv4.2CTX:DPPX-S channels. The QON-V relationships were described by assuming a Boltzmann function (see Materials and methods) with the following best-fit parameters (Table I): V1/2 (Kv4.2 CTX) = −47 mV and z (Kv4.2 CTX) = 2.98 e0 (n = 6); and V1/2 (Kv4.2CTX:DPPX-S) = −73 mV and z (Kv4.2CTX:DPPX-S) = 3.04 e0 (n = 7). Current traces were sampled in 2-mV increments. The mean Qmax values in the absence and presence of DPPX-S were 107 ± 28 fC/pF (n = 6) and 98 ± 21 fC/pF (n = 7), respectively. Data are expressed as mean ± SE.

Mentions: A previous study demonstrated that CTX effectively blocks Shaker Kv1 channels but has no effect on the Ig (Schoppa and Sigworth, 1998). Thus, to isolate the Kv4.2CTX Ig, the cells were exposed to saturating concentrations of CTX (∼100 nM), which completely eliminates the Kv4.2CTX ionic current (Fig. 1 C). Under these conditions and in response to step depolarizations, the ON and OFF components of the Ig were observed clearly (Fig. 2, A and B). The peak amplitude of the Ig-ON was typically >1 nA at the more depolarized membrane potentials. Qualitatively, the Kv4.2CTX Ig exhibits voltage dependence and kinetics that are characteristic of Kv channels (Bezanilla, 2000). The gating charge increases sharply with depolarization (Fig. 2 and Fig. 3, A and B), and consistent with the capacitive nature of the Ig, the ON and OFF gating charge–voltage relations (QON-V and QOFF-V relations, respectively) are equal (i.e., the gating charge is conserved; Fig. 3, C and D). Also, the overall kinetics of the Ig-ON became faster with membrane depolarization (see next paragraph; Fig. 2). The effects of DPPX-S on the Ig were striking. Coexpression of Kv4.2CTX and DPPX-S resulted in faster Ig-ON and Ig-OFF (Fig. 2, A and B) and a parallel leftward shift of the QON-V relationship (Fig. 4 and Table I). The magnitude of this shift (−26 mV) is comparable to that of the Gp-V relationship, as described in the previous section (Fig. 1 E and Table I).

Dougherty K, Covarrubias M - J. Gen. Physiol. (2006)

Bottom Line: Dipeptidyl aminopeptidase-like proteins (DPLPs) interact with Kv4 channels and thereby induce a profound remodeling of activation and inactivation gating.This shift is associated with faster outward movements of the gating charge over a broad range of relevant membrane potentials and accelerated gating charge return upon repolarization.In sharp contrast, DPPX-S had no effect on gating charge movements of the Shaker B Kv channel.

Affiliation: Department of Pathology, Anatomy and Cell Biology, Jefferson Medical College of Thomas Jefferson University, Philadelphia, PA 19107, USA.

Abstract: Dipeptidyl aminopeptidase-like proteins (DPLPs) interact with Kv4 channels and thereby induce a profound remodeling of activation and inactivation gating. DPLPs are constitutive components of the neuronal Kv4 channel complex, and recent observations have suggested the critical functional role of the single transmembrane segment of these proteins (Zagha, E., A. Ozaita, S.Y. Chang, M.S. Nadal, U. Lin, M.J. Saganich, T. McCormack, K.O. Akinsanya, S.Y. Qi, and B. Rudy. 2005. J. Biol. Chem. 280:18853-18861). However, the underlying mechanism of action is unknown. We hypothesized that a unique interaction between the Kv4.2 channel and a DPLP found in brain (DPPX-S) may remodel the channel's voltage-sensing domain. To test this hypothesis, we implemented a robust experimental system to measure Kv4.2 gating currents and study gating charge dynamics in the absence and presence of DPPX-S. The results demonstrated that coexpression of Kv4.2 and DPPX-S causes a -26 mV parallel shift in the gating charge-voltage (Q-V) relationship. This shift is associated with faster outward movements of the gating charge over a broad range of relevant membrane potentials and accelerated gating charge return upon repolarization. In sharp contrast, DPPX-S had no effect on gating charge movements of the Shaker B Kv channel. We propose that DPPX-S destabilizes resting and intermediate states in the voltage-dependent activation pathway, which promotes the outward gating charge movement. The remodeling of gating charge dynamics may involve specific protein-protein interactions of the DPPX-S's transmembrane segment with the voltage-sensing and pore domains of the Kv4.2 channel. This mechanism may determine the characteristic fast operation of neuronal Kv4 channels in the subthreshold range of membrane potentials.