Submitted on October 18, 2006
Accepted on December 16, 2006
Identification of a Novel Signal in the Cytoplasmic Tail of the Na+:HCO3- Cotransporter NBC1 That Mediates Basolateral Targeting
Hong Chao Li1, Emily Yi Li1, Lisa Neumeier1, Laura Conforti1, and Manoocher Soleimani1*
1 Internal Medicine, University of Cincinnati, Cincinnati, Ohio, United States
* To whom correspondence should be addressed. E-mail: manoocher.soleimani{at}uc.edu.
The Na+:HCO3- cotransporter NBC1 (SLC4A4, variant A, kidney specific) is located exclusively on the basolateral membrane of epithelial cells, implying that this molecule has acquired specific signals for targeting to the basolateral membrane. A motif with the sequence QQPFLS (positions 1010-1015) in the cytoplasmic tail of NBC1 was recently demonstrated to mediate targeting of NBC1 to the basolateral membrane. Here we demonstrate that mutating the amino acid F (Phenylalanine) or L (Leucine) at positions 1013 or 1014 to Alanine, respectively, resulted in the retargeting of NBC1 to the apical membrane. Further, mutation of the FL motif to FF showed similar properties as the wild-type, however, mutation of the FL motif to LL showed significant intracellular retention of NBC1. Mutating the amino acids Q-Q-P and S (positions 1010-1011-1012 and 1015) to A-A-A and A, respectively, did not affect the membrane targeting of NBC1. Functional studies in oocytes with microelectrode demonstrated that the apically-targeted mutants, as well as basolaterally targeted mutants, are all functional. We propose that the FL motif in the carboxyl terminal tail of NBC1 is essential for the targeting of NBC1 to the basolateral membrane, but is distinct from the membrane targeting di-leucine motif identified in other membrane proteins.
