Figure 1.

Topological representation of the rat type IIa sodium phosphate cotransporter (rat NaPi-IIa). Scheme is based on current hydropathy data and a recent topology study (Lambert et al. 1999) in which eight membrane spanning regions (TM1–TM8) are predicted with extracellular loops between TM1–TM2, TM3–TM4, TM5–TM6, and TM7–TM8, and both the carboxy and amino termini are intracellular. (•) Positions of 15 residues that were individually mutated to cysteines (see methods and materials).