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Published on September 17, 2008; 10.1104/pp.108.129478

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Received September 8, 2008
Accepted September 15, 2008

Functional characterization of the plastidic phosphate translocator gene family from the thermo-acidophilic red alga Galdieria sulphuraria reveals specific adaptations of primary carbon partitioning in green plants and red algae

Marc Linka , Aziz Jamai , and Andreas P.M. Weber *

Institut fur Biochemie der Pflanzen, Heinrich-Heine-Universitat, 40225 Dusseldorf, Germany; Genetics Graduate Program, Michigan State University, East Lansing, MI 48824; Botanisches Institut II, Albertus-Magnus-Universitat, 50931 Koln, Germany; Current Address: Department of Biological Sciences, Dartmouth College, Hanover, NH 03755

* Corresponding author; email: andreas.weber{at}uni-duesseldorf.de.

In chloroplasts of green plants and algae, CO2 is assimilated into triose-phosphates; a large part of these triose-phosphates is exported to the cytosol by a triose-phosphate/phosphate translocator (TPT) whereas some is stored in the plastid as starch. Plastidial phosphate translocators (PTs) have evolved from transport proteins of the host endomembrane system shortly after the origin of chloroplasts by endosymbiosis. The red microalga Galdieria sulphuraria shares three conserved putative orthologous transport proteins with the distantly related seed plants and green algae. However, red algae, in contrast to green plants, store starch in their cytosol, not inside plastids. Hence, due to the lack of a plastidic starch pool, a larger share of recently assimilated CO2 needs to be exported to the cytosol. We thus hypothesized that red algal transporters have distinct substrate specificity in comparison to their green orthologs. This hypothesis was tested by expression of the red algal genes in yeast and assessment of their substrate specificities and kinetic constants. Indeed, two of the three red algal PT candidate orthologs have clearly distinct substrate specificities when compared to their green homologs. GsTPT displays very narrow substrate specificity and high affinity; in contrast to green plant TPTs, 3-phosphoglyceric acid is poorly transported and thus not able to serve as a triose phosphate/3-PGA redox shuttle in vivo. Apparently, the specific features of red algal primary carbon metabolism promoted the evolution of a highly efficient export system with high affinities for its substrates. The low-affinity TPT of plants maintains triose phosphate levels sufficient for starch biosynthesis inside of chloroplasts, whereas the red algal TPT is optimized for efficient export of triose phosphate from the chloroplast.






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