Phosphatase as Starch Control

Abstract

Plant metabolism shifts dramatically with the daily light cycle, from a situation in which carbon is fixed into sugars and stored as starch during photosynthesis to one in which reserves of starch are used for energy during the night. Precisely how this switch from biosynthesis to consumption is regulated has not been clear. But now Sokolov et al. have characterized an enzyme that may couple metabolism of starch to daily cycles. Taking a cue from the knowledge that in animals, accumulation of glycogen is regulated by a dual-specificity phosphatase (that is, one that modifies both phosphotyrosine and phosphoserine or phosphothreonine residues), the authors characterized a related phosphatase that they call DSP4. Mutant Arabidopsis plants in which the DSP4 gene was altered showed increased accumulation of starch in the leaves. DSP4 also interacted directly with starch granules in in vitro binding assays and, in plants expressing DSP4 tagged with green fluorescent protein, DSP4 was associated with starch granules during the day but not at night. The pH of the chloroplast stroma increases from about pH7 at night to about pH8 in the daytime. The redox environment also changes, with various enzymes being reduced during the day. The authors propose that these signals may couple to control of starch metabolism through DSP4 because DSP4 activity in in vitro assays was lost as pH increased from pH7 to pH8. Addition of oxidants decreased binding of DSP4 to starch granules and decreased the activity of the phosphatase. Thus, plants may have a phosphorylation-mediated mechanism controlling storage of energy in starch that resembles that of animals for control of glycogen metabolism. A related paper recently appeared in the Journal of Biological Chemistry . L. N. Sokolov, J. R. Dominguez-Solis, A.-L. Allary, B. B. Buchanan, S. Luan, A redox-regulated chloroplast protein phosphatase binds to starch diurnally and functions in its accumulation. Proc. Natl. Acad. Sci. U.S.A. 103 , 9732-9737 (2006). [Abstract] [Full Text] T. Niittylä, S. Comparot-Moss, W.-L. Lue, G. Messerli, M. Trevisan, M. D. J. Seymour, J. A. Gatehouse, D. Villadsen, S. M. Smith, J. Chen, S. C. Zeeman, A. M. Smith, Similar protein phosphatases control starch metabolism in plants and glycogen metabolism in mammals. J. Biol. Chem. 281 , 11815-11818 (2006). [Abstract] [Full Text]