Plant and Cell Physiology Advance Access originally published online on June 7, 2006
Plant and Cell Physiology 2006 47(7):959-971; doi:10.1093/pcp/pcj068
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The Three Maize Sucrose Synthase Isoforms Differ in Distribution, Localization, and Phosphorylation
1 Department of Plant Biology, University of Illinois Urbana Champaign, Urbana, IL 61801, USA
2 Program in Physiological and Molecular Plant Biology, University of Illinois Urbana Champaign, Urbana, IL 61801, USA
3 United States Department of Agriculture-Agricultural Research Service Photosynthesis Research Unit and Department of Crop Sciences, University of Illinois Urbana Champaign, Urbana, IL 61801, USA
* Corresponding author: E-mail, schuber1{at}life.uiuc.edu; Fax, +1-217-244-4419.
Although sucrose synthase (SUS) is widely appreciated for its role in plant metabolism and growth, very little is known about the contribution of each of the SUS isoforms to these processes. Using isoform-specific antibodies, we evaluated the three known isoforms individually at the protein level. SUS1 and SUS-SH1 proteins have been studied previously; however, SUS2 (previously known as SUS3) has only been studied at the transcript level. Using SUS2 isoform-specific antibodies, we determined that this isoform is present in several maize tissues. The intracellular localization of all SUS isoforms was studied by cellular fractionation of leaves and developing kernels. Interestingly, SUS1 and SUS-SH1 were associated with membranes while SUS2 was not. The lack of membrane-associated SUS2 indicates that it might have a unique role in cytoplasmic sucrose metabolism. Using co-immunoprecipitation with kernel extracts, it was also established that SUS2 exists predominantly as a hetero-oligomer with SUS1, while SUS-SH1 forms only homo-oligomers. Using sequence-specific and phospho-specific antibodies, we have established for the first time that SUS-SH1 is phosphorylated in vivo at the Ser10 site in kernels, similar to the SUS1 Ser15 site. In midveins, additional evidence suggests that SUS can be phosphorylated at a novel C-terminal threonine site. Together, these results show that the isoforms of SUS are important in both cytosolic and membrane-associated sucrose degradation, but that their unique attributes most probably impart isoform-specific functional roles.
(Received March 29, 2006; Accepted May 24, 2006)
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