A Chloroplast-localized Dual-specificity Protein Phosphatase in Arabidopsis Contains a Phylogenetically Dispersed and Ancient Carbohydrate-binding Domain, Which Binds the Polysaccharide Starch

Dual-specificity protein phosphatases (DSPs) are important regulators of a wide variety of protein kinase signaling cascades in animals, fungi and plants. We previously identified a cluster of putative DSPs in Arabidopsis (including At3g52180 and At3g01510) in which the phosphatase domain is related to that of laforin, the human protein mutated in Lafora epilepsy. In animal and fungal systems, the laforin DSP and the beta-regulatory subunits of AMP-regulated protein kinase (AMPK) and Snf-1 have all been demonstrated to bind to glycogen by a glycogen-binding domain (GBD). We present a bioinformatic analysis which shows that these DSPs from Arabidopsis, together with other related plant DSPs, share with the above animal and fungal proteins a widespread and ancient carbohydrate-binding domain. We demonstrate that DSP At3g52180 binds to purified starch through its predicted carbohydrate-binding region, and that mutation of key conserved residues reduces this binding. Consistent with its ability to bind exogenous starch, DSP At3g52180 was found associated with starch purified from Arabidopsis plants and suspension cells. Immunolocalization experiments revealed a co-localization with chlorophyll, placing DSP At3g52180 in the chloroplast. Gene-expression data from different stages of the light-dark cycle and across a wide variety of tissues show a strong correlation between the pattern displayed by transcripts of the At3g52180 locus and that of genes encoding key starch degradative enzymes. Taken together, these data suggest the hypothesis that plant DSPs could be part of a protein assemblage at the starch granule, where they would be ideally situated to regulate starch metabolism through reversible phosphorylation events.