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Published on August 20, 2008; 10.1104/pp.108.126862

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Received July 22, 2008
Accepted August 8, 2008

Characterization of an autonomously activated plant adenosine diphosphate glucose pyrophosphorylase

Susan K. Boehlein , Janine R. Shaw , Jon D. Stewart , and L. Curtis Hannah *

Program in Plant Molecular and Cellular Biology and Horticultural Sciences, University of Florida, Gainesville, Florida, 32611; Department of Chemistry, University of Florida, Gainesville, Florida, USA 32611-7200

* Corresponding author; email: Hannah{at}mail.ifas.ufl.edu.

ADP-glucose pyrophosphorylase (AGPase) catalyzes the rate limiting step in starch biosynthesis in plants and changes in its catalytic and/or allosteric properties can lead to increased starch production. Recently, a maize/potato small subunit mosaic, MP [Mos(1-198)], containing the first 198 amino acids of the small subunit of the maize endosperm enzyme and the last 277 amino acids from the potato tuber enzyme was expressed with the maize endosperm large subunit and was reported to have favorable kinetic and allosteric properties. Here we show that this mosaic, in the absence of activator, performs like a wild-type (wt) AGPase that is partially activated with 3-PGA. In the presence of 3-PGA, enzyme properties of Mos(1-198)/SH2 are quite similar to those of the wt maize enzyme. In the absence of 3-PGA, however, the mosaic enzyme exhibits greater activity, higher affinity for the substrates and partial inactivation by Pi. The Mos(1-198)/SH2 enzyme is also more stable to heat inactivation. The different properties of this protein were mapped using various mosaics containing smaller portions of the potato small subunit. Enhanced heat stability of Mos(1-198) was shown to originate from five potato-derived amino acids between 322 and 377. These amino acids were shown previously to be important in small subunit/large subunit interactions. These five potato-derived amino acids plus other potato-derived amino acids distributed throughout the carboxyl terminal portion of the protein are required for the enhanced catalytic and allosteric properties exhibited by Mos(1-198)/SH2.




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