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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Both Subunits of ADP-Glucose Pyrophosphorylase Are Regulatory¹

Department of Horticultural Sciences (J.M.C., M.C., J.R.S., L.C.H.) and Department of Microbiology and Cell Science (R.R.S.), Program in Plant Molecular and Cellular Biology, University of Florida, Gainesville, Florida 32611; Dow Agroscience LLC, Indianapolis, Indiana 46268 (T.W.G.); and Institute of Biological Chemistry, Washington State University, Pullman, Washington 99164–6340 (T.W.O.)

The allosteric enzyme ADP-Glc pyrophosphorylase (AGPase) catalyzes the synthesis of ADP-Glc, a rate-limiting step in starch synthesis. Plant AGPases are heterotetramers, most of which are activated by 3-phosphoglyceric acid (3-PGA) and inhibited by phosphate. The objectives of these studies were to test a hypothesis concerning the relative roles of the two subunits and to identify regions in the subunits important in allosteric regulation. We exploited an Escherichia coli expression system and mosaic AGPases composed of potato (Solanum tuberosum) tuber and maize (Zea mays) endosperm subunit fragments to pursue this objective. Whereas potato and maize subunits have long been separated by speciation and evolution, they are sufficiently similar to form active mosaic enzymes. Potato tuber and maize endosperm AGPases exhibit radically different allosteric properties. Hence, comparing the kinetic properties of the mosaics to those of the maize endosperm and potato tuber AGPases has enabled us to identify regions important in regulation. The data herein conclusively show that both subunits are involved in the allosteric regulation of AGPase. Alterations in the small subunit condition drastically different allosteric properties. In addition, extent of 3-PGA activation and extent of 3-PGA affinity were found to be separate entities, mapping to different regions in both subunits.

² Present address: Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, 14476 Golm, Germany.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.036699.

^* Corresponding author; e-mail hannah{at}ifas.ufl.edu; fax 352–392–6957.

Received November 25, 2003; returned for revision March 1, 2004; accepted March 5, 2004.

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