Masuhide Yakehiro

On site of action of grayanotoxin in domain 4 segment 6 of rat skeletal muscle sodium channel

Grayanotoxin I (GTX I) is a diterpenoid extracted from the family of Ericaceae that binds to Na+ channels and causes persistent activation. We investigated the interaction of GTX I with the amino acid residues I1575, F1579 and Y1586 in transmembrane segment D4S6 of μ1. In F1579A, GTX shifted the threshold potential about 50 mV in the hyperpolarizing direction and modified Na+ channels twice as efficiently as that in wild‐type. In contrast, these GTX‐effects were eliminated completely in the I1575A mutant and were reduced substantially in mutant Y1586A. Lysine substitution for F1579 significantly reduced and for Y1586 completely eradicated the GTX‐effect. Our data suggest that the GTX receptor site shares overlapping but non‐identical molecular determinants with BTX in D4S6 and has common molecular determinants in D1S6.

State-dependent action of grayanotoxin I on Na + channels in frog ventricular myocytes

1 Distinct properties of grayanotoxin (GTX) among other lipid‐soluble toxins were elucidated by quantitative analysis made on the Na+ channel in frog ventricular myocytes. 2 GTX‐modified current (IGTX) was induced strictly in proportion to the open probability of Na+ channels during preconditioning pulses irrespective of its duration, amplitude or partial removal of inactivation by chloramine‐T. This confirms that GTX binds to the Na+ channel exclusively in its open state, while batrachotoxin (BTX) was reported to be capable of modifying slow‐inactivated Na+ channels, and veratridine exhibited voltage‐dependent modification. 3 The GTX‐modified channel did not show any inactivation property, which is different from reported results with veratridine and BTX. 4 Estimated unbinding rates of GTX were in reverse proportion to the activation curve of GTX‐modified Na+ channels. This was not the previously reported case with veratridine. 5 A model including unbinding kinetics of GTX and slow inactivation of unmodified Na+ channels in which GTX was permitted to bind only to the open state of Na+ channels indicated that unbinding reactions of GTX occur only in the closed state.

Structural determinants for the action of grayanotoxin in D1 S4.S5 and D4 S4.S5 intracellular linkers of sodium channel α-subunits

We located a novel binding site for grayanotoxin on the cytoplasmic linkers of voltage-dependent cardiac (rH1) or skeletal-muscle (mu 1) Na(+) channel isoforms (segments S4-S5 in domains D1 and D4), using the alanine scanning substitution method. GTX-modification of Na(+) channels, transiently expressed in HEK 293 cells, was evaluated under whole-cell voltage clamp, from the ratio of maximum chord conductance for modified and unmodified Na(+) channels. In mu 1, mutations K237A, L243A, S246A, K248A, K249A, L250A, S251A, or T1463A, caused a moderate, but statistically significant decrease in this ratio. On making corresponding mutations in rH1, only L244A dramatically reduced the ratio. Because in mu 1, the serine at position 251 is the only heterologous residue with respect to rH1 (Ala-252), we made a double mutant L243A&S251A to match the sequence of mu 1 and rH1 in S4-S5 linkers of both domains. This double mutation resulted in a significant decrease in the ratio, to the same extent as L244A substitution in rH1 did, indicating that the site at Leu-244 in rH1 or at Leu-243 in mu 1 is a novel one, exhibiting a synergistic effect of grayanotoxin.