Alternative topological model of human PiT2 and mutants. Membrane topology model for human PiT2 suggested by Salaün and coworkers ; the TMs are shown as grey-filled sequences and their numbers are indicated with roman numbers above the model. This model shares some similarity to a membrane topology model for PiT1 proposed in 2002 . Based on the cellular location of C-terminal tags, the C-terminal ends of PiT1 and PiT2 were predicted to be extracellular [22, 24]. And based on the cellular location of an N-terminal tag on PiT2 and glycosylation of a site in human PiT1 and partly glycosylation of the same site in human PiT2 although oddly not in hamster PiT2, the N-termini of PiT1 and PiT2 were suggested to be extracellular [22, 24]; due to a suggested additional TM after TM3 in Figure 1 (TMIV in this figure), this did not influence the orientation of the large intracellular domain in these models compared to the model in Figure 1. The PiT2 model shown in Figure 1 and this figure, respectively, and the PiT1 model proposed in 2002  were later compared by us . In 2009, Farrell and coworkers proposed a modified model of human PiT1 based on substituted cysteine accessibility mutagenesis . The recent model of PiT1 shows more resemblance to the PiT2 models shown in this figure and in Figure 1 concerning the length and position of the large intracellular domain (L6) than the model from 2002. The amino acids identified in human PiT2 and human PiT1 as being critical for Pi transport function are highlighted with black filling and pointed out with arrows; for references see legend to Figure 1. Compared to the PiT2 model in Figure 1, the PiT2 model proposed by Salaün and coworkers (this figure) and the PiT1 model proposed in 2009 by Farrell and coworkers do not affect the placement of PiT1 D43 (PiT2 D28), PiT1 E70 (PiT2 E55), PiT1 H530 (PiT2 H502), PiT1 D534 (PiT2 D506), PiT1 E603 (PiT2 E575), and PiT1 S621 (PiT2 S593) in either a TM domain or a loop sequence [23, 24] (compare Figure 1 and this figure). However, PiT1 S128 (PiT2 S113) placed in loop regions in the PiT2 models (this figure and Figure 1), is in the PiT1 model from 2009 suggested to be placed in a TM domain .