Similar protein phosphatases control starch metabolism in plants and glycogen metabolism in mammals

We report that protein phosphorylation is involved in the control of starch metabolism in Arabidopsis leaves at night. sex4 (starch excess 4) mutants, which have strongly reduced rates of starch metabolism, lack a protein predicted to be a dual specificity protein phosphatase. We have shown that this protein is chloroplastic and can bind to glucans and have presented evidence that it acts to regulate the initial steps of starch degradation at the granule surface. Remarkably, the most closely related protein to SEX4 outside the plant kingdom is laforin, a glucan-binding protein phosphatase required for the metabolism of the mammalian storage carbohydrate glycogen and implicated in a severe form of epilepsy (Lafora disease) in humans.

Starch, the main storage carbohydrate of plants, accumulates as a product of photosynthesis in leaves during the day and is converted to sucrose for export from the leaves at night. This conversion of starch to sucrose is one of the largest daily carbon fluxes on the planet, but nothing is known about how the process is initiated and controlled. The amounts of enzymes on the pathway change very little through the diurnal cycle in leaves of the model plant Arabidopsis thaliana, hence flux must be controlled by modulation of their activities (1).

Much progress in understanding the pathway has been made through the selection of Arabidopsis mutants impaired in starch degradation at night. All such mutations identified thus far are in genes encoding enzymes of the pathway, rather than proteins likely to be involved in modulation of the activities of these enzymes (2-11). However, a mutation at a locus not yet identified, the starch excess 4 (or SEX4) locus, gives rise to a phenotype indicative of a regulatory defect rather than a defect in a structural enzyme. Mature sex4 leaves contain three to four times more starch than those of wild-type plants, apparently because a reduced capacity for starch degradation at night leads to progressive accumulation of starch over the life of the leaf (12, 13). Starch granules in leaves of the sex4 mutant are much larger and more rounded than those of wild-type plants (14). Measurements of activity and protein of enzymes known to be involved in starch degradation revealed only one significant reduction in the sex4 mutant in the chloroplastic α-amylase AMY3 (12, 15). However, although both the activity and amount of protein of AMY3 are strongly reduced, this is not the cause of the deficiency in starch degradation in the sex4 mutant. T-DNA insertion lines lacking AMY3 protein have normal rates of starch degradation (15). The aim of the work described in this paper was to discover the nature of the gene at the SEX4 locus and thus shed light on the regulation of starch degradation.