The first external loop of the metal ion transporter XXXX is involved in metal ion binding and specificity.

@entity1 @entity8862 @entity8798 @entity1238 @entity535 @entity73018 @entity4527 @entity3281 @entity39605 @entity73019 @entity45241 @entity375
The @entity375 null mutant smf1Delta cannot grow on medium containing @entity4527 . Expression of Smf1p or the @entity1 transporter @entity39605 ( @entity39605 ) suppresses the above-mentioned phenotype. Both can also be expressed in @entity1238 oocytes, and the uptake activity and their electrophysiological properties can be studied. We used these systems to analyze the properties of mutations in the predicted external loop I of @entity39605 . The sensitivity of the transporter to @entity535 substitutions in this region is manifested by the mutation @entity45241 , which resulted in almost complete inhibition of the metal ion uptake activity and marked changes in the pre-steady-state currents in @entity1238 oocytes. The mutation @entity73018 abolished the uptake and the electrophysiology, but the double mutant @entity73019 / @entity73018 partially restored it and changed the metal ion specificity in favor of @entity8798 . The maximal pre-steady-state currents at negatively imposed potentials shifted to a lower pH of approximately 5. The triple mutant @entity45241 / @entity73019 / @entity73018 , which has no apparent transport activity, exhibited remarkable pre-steady-state currents at pH 7.5. Moreover, @entity8862 had a dual effect on this mutant; at pH 7.5 it eliminated the pre-steady state without generating steady-state currents, and at pH 5.5 it induced large pre-steady-state currents. The mutant @entity73019 retained appreciable @entity8798 uptake activity but exhibited very little @entity3281 uptake at pH 5.5 and was abolished at pH 6.5. The properties of the various mutants suggest that loop I is involved in the metal ion binding and its coupling to the proton-driving force.
