Requirement of a leucine residue for (apical) membrane expression of type IIb NaPi cotransporters

Type II NaPi cotransporters mediate epithelial phosphate (P-i) reabsorption . In mammals the type IIb protein is expressed in the small intestinal apic al membrane and other epithelia; it is not expressed in the renal proximal tubule where we find the type IIa isoform, To look for molecular determinan t(s) involved in apical expression of type IIb cotransporters, we have made deletion mutations within the C-terminal tails of mouse IIb (mIIb) and hum an IIb (hIIb) transporter proteins. The constructs were fused to the enhanc ed green fluorescent protein and transiently transfected into intestinal Ca Co2-cells, Both mIIb and hIIb were located exclusively in the apical membra ne of the cells. For mIIb, the removal of a cysteine cluster or the last th ree amino acids (TVF) had no effect on the location of the protein. However , truncation at the level of the conserved L691/689 prevented the apical me mbrane expression of both mIIb and hIIb, respectively, and the mutated prot eins were located in endosomal and lysosomal structures. A similar expressi on pattern of the mIIb and hIIb constructs was found in renal proximal tubu lar opossum kidney cells. Our data suggest that L691/689 is involved in mec hanisms leading to an apical expression of type IIb NaPi cotransporters.