Shaker potassium channels undergo a slow C-type inactivation which can
be hastened dramatically by single-point mutations in or near the por
e region. We found that the oxidizing agent chloramine-T (Chl-T) cause
s an irreversible loss of current for those mutants which show C-type
inactivation. For several mutants at position T449, which show a wide
spectrum of inactivation time constants, the time constant of current
rundown induced by Chl-T correlated with the speed of inactivation. Ru
ndown was accelerated when the channels were in the inactivated state
but rundown also occurred when channels were not opened or inactivated
. Apparently, only those channels which can undergo C-type inactivatio
n are accessible to Chl-T. In order to gain information about the targ
et amino-acid residue for the action of Chl-T and the structural rearr
angements occurring during C-type inactivation, several mutant channel
proteins were compared with respect to their response to Chl-T. Since
Chl-T can oxidize cysteine and methionine residues, we mutated the po
ssible targets in and close to the pore region, namely C462 to A, and
M440 and M448 to I. While the residues M440 and C462 were not importan
t for channel rundown, mutation of M448 to I made the channels more re
sistant to Chl-T by about one order of magnitude. While inactivation w
as accelerated upon application of Chl-T in most mutants, mutation of
M448 to I abolished this effect on the time course of inactivation, in
dicating that M448 is one of the target residues for Chl-T.