The  thylakoid   anion  transporter 1  (ANTR1)   from  Arabidopsisthaliana,  has been characterized as a Na-dependent Pi transporter when expressed in E. coli (1), but  no data  is yet available  for the protein  structure  and  amino  acids involved in transport of Pi. In this  study  a  three-dimensional structural  model  of  ANTR1  was constructed in silico using the crystal structure  of glycerol-3- phosphate/phosphate antiporter from E. coli as a template.  Based on Multiple  Sequence Alignments (MSAs) with other plant  ANT- Rs  and  mammalian   SLC17  homologues,   five  highly  conserved amino  acids involved in Pi transport have been identified,  namely Arg-120, Ser-124 and Arg-201 inside the putative translocation pathway,  Arg-228  and  Asp-382  exposed  at  the  cytoplasmic  sur- face of the protein.  The activity of the protein  as a Na-dependent Pi transporter in the wild type and mutants  was analyzed  by het- erologous  expression  and  uptake   of  radioactive   Pi  into  E.  coli cells. Substitution of the three Arg (120, 201 and 228) for Glu residues  and  of Asp-382 for  an  Asn residue  resulted  in an  inac- tive ANTR1  transporter. All other  mutants  had sufficient activity to  allow  measurement   of  kinetic  parameters, attesting   that  the mutated  proteins  were functional.  Based on  our  results,  we pro- pose that Arg-201 is a critical residue for substrate  binding and translocation, whereas Ser-124 may function  as periplasmic  gate- way for  Na+   ions.  Residue  Arg-120  plays  an  important role  in Pi  binding  and  associated   conformational  changes,  and  finally that Arg-228 and Asp-382 only weakly participate  in interactions allowing conformational changes to occur at the cytoplasmic  sur-face of the transporter.