Cloning and in vivo expression of functional triose phosphate/phosphate translocators from C3- and C4-plants: evidence for the putative participation of specific amino acid residues in the recognition of phosphoenolpyruvate.

The primary sequences of the chloroplast triose phosphate/phosphate translocator precursor proteins from C4-plants (maize mesophyll cells and Flaveria trinervia) and from the C3-type Flaveria pringlei were determined. The mature parts of these translocators possess 83-94% identical amino acid residues. The C4-translocator protein can be correctly targeted to C3-type chloroplasts and inserted into the envelope membrane. Expression of the mature parts of these chloroplast translocators (cTPT) in transformed yeast cells and subsequent reconstitution of the functional proteins reveals the difference between the recombinant translocator proteins from the two cell types with respect to the transport of phosphoenolpyruvate. Comparison of the cTPT sequences from F. pringlei and F. trinervia in combination with computer-aided molecular modelling of the substrate translocation pore leads to the suggestion, that only minor exchanges of amino acid residues between the C3- and C4-translocator proteins are sufficient to extend their substrate specificities to recognize also phosphoenolpyruvate.