Lorberth et al. describe in Nature Biotechnology (Vol.16 (5) - May 98) the effects on starch metabolism exerted if the expression of this gene is inhibited in transgenic potato plants using antisense technology. A major observation made is the reduction of the phosphate monoester content of the starch synthesized in the transgenic lines down to 10% as compared with wild-type plants. This indicates that the R1 protein is responsible for the phosphorylation of starch, which is also supported by the fact that the expression of the protein in Escherichia coli leads to elevated phosphate contents of the synthesized glycogen. The existance of the R1 protein is not confined to the Solanaceous species; corresponding sequences are also found in Arabidopsis and rice, but not in bacteria, mammals and yeasts, indicating that it is a general but unique component of starch biosynthesis.
The biochemical mechanism by which the R1 protein phosphorylates amylopectin remains to
be elucidated. It is not known which substrate acts as a phosphoryl donor, nor which type of
glucans are phosphate acceptors. Evidence that phosphoenolpyruvate may act as a phosphoryl
donor comes from slight sequence homology of the R1 protein to the PEP synthases from
different bacteria. The question of a phosphoryl acceptor is far more difficult to address, as
the different intermediates between ADP-glucose and amylope
Contact: Jens Kossmann