We have cloned a gene involved in starch metabolism that was identified by the ability of its product to bind to potato starch granules. Reduction in the protein level of transgenic potatoes leads to a reduction in the phosphate content of the starch. The complementary result is obtained when the protein is expressed in Escherichia coli, as this leads to an increased phosphate content of the glycogen. It is possible that this protein is responsible for the incorporation of phosphate into starch-like glucans, a process that is not understood at the biochemical level. The reduced phosphate content in potato starch has some secondary effects on its degradability, as the respective plants show a starch excess phenotype in leaves and a reduction in cold-sweetening in tubers.