Renouncing electroneutrality is not free of charge:: Switching on electrogenicity in a Na+-coupled phosphate cotransporter

Renal type IIa Na+-coupled inorganic phosphate (Pi) cotransporters (NaPi-IIa) mediate divalent Pi transport in an electrogenic manner, whereas the renal type IIc isoform (NaPi-IIc) is electroneutral, yet it shows high sequence identity with NaPi-IIa. Dual uptake ((32)p(i)/Na-22) assays confirmed that NaPi-IIc displayed Na+-coupled Pi cotransport with a 2:1 (Na+:Pi) stoichiometry compared with 3:1 established for NaPi-IIa. This finding suggested that the electrogenicity of NaPi-IIa arises from the interaction of an additional Na+ ion compared with NaPi-IIc. To identify the molecular elements responsible for the functional difference between isoforms, we used chimera and amino acid replacement approaches. Transport activity of chimeras constructed with NaPi-IIa and NaPi-IIc indicated that residues within the first six transmembrane domains were essential for the electrogenicity of NaPi-IIa. Sequence comparison between electrogenic and electroneutral isoforms revealed differences in the charge and polarity of residues clustered in three areas, one of which included part of the predicted third transmembrane domain. Here, substitution of three residues with their NaPi-Ila equivalents in NaPi-IIc (S189A, S191A, and G195D) resulted in a transporter that displayed a 1:1 charge/P-i coupling, a 3:1 Na+:P-i stoichiometry, and transient currents that resembled pre-steady-state relaxations. The mutant's weaker voltage dependency and 10-fold lower apparent Pi affinity compared with NaPi-Ila indicated that other residues important for the NaPi-Ila kinetic fingerprint exist. Our findings demonstrate that, through a minimal number of side chain substitutions, we can effect a switch from electroneutral to electrogenic cotransporter function, concomitant with the appearance of a cosubstrate interaction site.