The inner envelope membrane of chloroplasts and non-green plastids represents the major barrier for selective transport of metabolites between the stroma and the surrounding cytosol. However, the exchange of metabolites between both compartments is essential for the survival of plants. This task is fulfilled by metabolite transporters, specific membrane proteins, which form integral components of the inner envelope membrane.

Amongst the plastidial metabolite transporters, the phosphate translocators (PT) are unique in that they catalyse a strict counter exchange of phosphorylated metabolic intermediates with either inorganic phosphate or other phophorylated intermediates. The driving force for the transport derives from concentration gradients between the compartments.

The PT family belongs to the superfamily of nucleotide sugar transporters. To date four members of the PT family have been characterised with both distinct and partially overlapping substrate specificities:

Triose phosphate/phosphate translocator (TPT; Fliege et al., 1978; Flügge et. al., 1989)

Phosphoenolpyruvate/phosphate translocator (PPT; Fischer et al., 1997)

Glucose 6-phosphate/phosphate translocator (GPT; Kammerer et al., 1998)

Xylulose 5-phoshate/phosphate translocators (XPT; Eicks et al., 2002)

The metabolic sketch in Fig. 1 illustrates the multifaceted roles of PTs in the exchange of metabolites between stroma and cytosol both in photoautotrophic and heterotrophic tissues. Studies with mutant or transgenic plants delivered novel aspects of how transport processes interact with plant development, performance and signalling.

(back to Rainer E. Häusler)