Potassium channels are ubiquitous, being present in all living organisms. T
hese proteins share common structural elements, which confer common functio
nal features. In general, all K+ channels have a high selectivity for K+, a
nd are blocked by cations of similar dimensions, such as Cs+ and Ba2+. Muta
tions in the pore region tend to lead to either the total loss of function
or K+ selectivity, We have made mutations to one of the most highly conserv
ed residues of the pore, glycine-143, of the inward rectifier ROMKI (Kir1.1
), and examined the resulting channel properties in the Xenopus oocyte expr
ession system with a two-electrode voltage clamp. Mutations G143A and G143R
resulted in failure to express functional channels. Co-injection of wild-t
ype ROMKI cRNA with these mutants led to rescue of channel function, which
was different from wi id-type ROMKI. In both mutants, the sensitivity to Ba
2+ and Cs+ was increased, the rate of onset of block by Ba2+ was enhanced,
and the selectivity to potassium was reduced. Whereas the crystallographic
evidence shows that cations bind to the carbonyl backbone of the pore-linin
g residues, the present results indicate that the side chains of these amin
o acids, which face away from the pore lining, also affect permeation.