Heteropoda Toxin 2 Interaction with Kv4.3 and Kv4.1 Reveals Differences in Gating Modification

Abstract

Kv4 (Shal) potassium channels are responsible for the transient outward K ϩ currents in mammalian hearts and central nervous systems. Heteropoda toxin 2 (HpTx2) is an inhibitor cysteine knot peptide toxin specific for Kv4 channels that inhibits gating of Kv4.3 in the voltage-dependent manner typical for this type of toxin. HpTx2 interacts with four independent binding sites containing two conserved hydrophobic amino acids in the S3b transmembrane segments of Kv4.3 and the closely related Kv4.1. Despite these similarities, HpTx2 interaction with Kv4.1 is considerably less voltage-dependent, has smaller shifts in the voltage-dependences of conductance and steady-state inactivation, and a 3-fold higher K d value. Swapping four nonconserved amino acids in S3b between the two channels exchanges the phenotypic response to HpTx2. To understand these differences in gating modification, we constructed Markov models of Kv4.3 and Kv4.1 activation gating in the presence of HpTx2. Both models feature a series of voltagedependent steps leading to a final voltage-independent transition to the open state and closely replicate the experimental data. Interaction with HpTx2 increases the energy barrier for channel opening by slowing activation and accelerating deactivation. The greater degree of voltage-dependence in Kv4.3 occurs because it is the voltage-dependent transitions that are most affected by HpTx2; in contrast, it is the voltage-independent step in Kv4.1 that is most affected by the presence of toxin. These data demonstrate the basis for subtype-specificity of HpTx2 and point the way to a general model of gating modifier toxin interaction with voltage-gated ion channels.

Differential contribution of Kv4-containing channels to A-type, voltage-gated potassium currents in somatic and visceral dorsal root ganglion neurons

“…The inhibition of the Kv4.3 current by the toxin seemed to include both inhibitory mechanisms. A similar voltage-dependent and -independent blockade of Kv4.x currents has been reported recently with heteropoda toxin 2 (Desimone et al 2011). Similar to this report, we found that four amino acids at the end of S3 primarily determine voltage dependence of the toxin action.…”

Voltage Sensor Inactivation in Potassium Channels

“…Based on these findings, it would be of considerable interest to know what the tarantula toxins do to voltage sensor inactivation? As to that, it has been found that the toxins more specific for Kv4 channels, including Heteropoda toxins, cause a slowing of macroscopic inactivation, an acceleration of recovery from inactivation and a shift of steady-state inactivation curves to more positive potentials (Sanguinetti et al, 1997; Diochot et al, 1999; Escoubas et al, 2002; Ebbinghaus et al, 2004; DeSimone et al, 2009, 2011). These findings are in accordance with a prevention of the relaxed conformation of the voltage sensor because it is stabilized in its resting conformation.…”

Impact of KChIP2 on cardiac electrophysiology and the progression of heart failure

“…I to,f recovers from inactivation in the order of milliseconds, whereas I to,s recovers much slower. In order to inhibit I to,f , an inactivating pre-pulse to −40 mV for 50 ms can be applied, which captures I to,f in a closed state (Brouillette et al, 2004); alternatively Heteropoda toxin can be used, which selectively blocks K V 4 channels in low-micromolar concentrations (Sanguinetti et al, 1997; DeSimone et al, 2011; Liu et al, 2011). For evaluating I K,slow1 , the sensitivity toward 4-aminopyridine can be exploited, because I K,slow1 is sensitive in the micromolar range while I to,f , I to,s , and I K,slow2 are sensitive in the millimolar range (Guo et al, 1999).…”