Jp. Benitah et al., Molecular dynamics of the sodium channel pore vary with gating: Interactions between P-segment motions and inactivation, J NEUROSC, 19(5), 1999, pp. 1577-1585
Disulfide trapping studies have revealed that the pore-lining (P) segments
of voltage-dependent sodium channels undergo sizable motions on a subsecond
time scale. Such motions of the pore may be necessary for selective ion tr
anslocation. Although traditionally viewed as separable properties, gating
and permeation are now known to interact extensively in various classes of
channels. We have investigated the interaction of pore motions and voltage-
dependent gating in mu 1 sodium channels engineered to contain two cysteine
s within the P segments. Rates of catalyzed internal disulfide formation (k
(ss)) were measured in K1237C+W1531C mutant channels expressed in oocytes.
During repetitive voltage-clamp depolarizations, increasing the pulse durat
ion had biphasic effects on the k(ss), which first increased to a maximum a
t 200 msec and then decreased with longer depolarizations. This result sugg
ested that occupancy of an intermediate inactivation state (I-M) facilitate
s pore motions. Consistent with the known antagonism between alkali metals
and a component of slow inactivation, k(ss) varied inversely with external
[Na+](o). We examined the converse relationship, namely the effect of pore
flexibility on gating, by measuring recovery from inactivation in Y401C + E
758C (YC/EC) channels. Under oxidative conditions, recovery from inactivati
on was slower than in a reduced environment in which the spontaneous YC/EC
cross-link is disrupted. The most prominent effects were slowing of a compo
nent with intermediate recovery kinetics, with diminution of its relative a
mplitude. We conclude that occupancy of an intermediate inactivation state
facilitates motions of the P segments; conversely, flexibility of the P seg
ments alters an intermediate component of inactivation.