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

Wereport 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, themostclosely related protein to SEX4 outside the plant kingdom is laforin, a glucanbinding protein phosphatase required for the metabolism of the mammalian storage carbohydrate glycogen and implicated in a severe form of epilepsy (Lafora disease) in humans.