Figure 1.

Putative topological model of human PiT2 and mutants. Putative membrane topology model of human PiT2 on which the mutant proteins investigated in the present paper are based; the model was originally proposed by O'Hara and coworkers [8,20]. The numbers of the TMs are indicated above the model. Other membrane topology models have been proposed for PiT1 [22,23] and PiT2 [24], which suggested diverging topology for the two paralogs; the alternative PiT2 model is shown in Figure 2. The amino acids previously identified in human PiT2 as being critical for P_i transport function are highlighted with black filling and pointed out with arrows; D₂₈, E₅₅, S₁₁₃, D₅₀₆, E₅₇₅, and S₅₉₃[18,27,28]. In human PiT1, the amino acids S₁₂₈ (PiT2 S₁₁₃) and S₆₂₁ (PiT2 S₅₉₃) have previously been identified as being critical for PiT1 P_i transport function [29]. In the present study, human PiT2 H₅₀₂ (situated in the PiT family signature sequence) and human PiT1 E₇₀ (equivalent in position to human PiT2 E₅₅) are also identified as critical for P_i transport function (see Figure 3). The grey-filled sequences (L₁₁-L₁₆₁ and V₄₉₂-V₆₄₀), represent the N- and C-terminal, respectively, ProDom domains (PD001131) published in 2004 defining the PiT family members [27]. The dark grey-filed sequence (I₅₃-L₁₂₇) represents the most recent ProDom domain defining the PiT family members http://prodom.prabi.fr/.