The Three Maize Sucrose synthase Isoforms Differ in Distribution, Localization, and Phosphorylation

Authors: DUNCAN, KATERI - UNIVERSITY OF ILLINOIS; HARDIN, SHANE - UNIVERSITY OF ILLINOIS; Huber, Steven

Submitted to: Plant Cell Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2006
Publication Date: 7/1/2006
Citation: Duncan, K.A., Hardin, S.C., Huber, S.C. 2006. The Three Maize Sucrose synthase Isoforms Differ in Distribution, Localization, and Phosphorylation. Plant Cell Physiology. 47:959-971.

Interpretive Summary: There are three isoforms of sucrose synthase known in maize: SUS1, SUS-SH1, and SUS2 (previously called SUS3), which are encoded by the Sus1, Sh1, and Sus2 genes, respectively. The SUS2 isoform has only been characterized at the mRNA level and thus, nothing is known about this newly discovered isoform at the protein level. The relative distribution of each of the three isoforms across maize tissues has also not been evaluated to date. In the present study, we developed isoform-specific antibodies to quantitate the abundance of the three gene products, and also assess their association with cellular membranes. Moreover, we could also investigate which isoforms came together to produce the native form of the enzyme, which is generally considered to be a tetramer (composed of four subunits). Interestingly, the results showed that SUS2 does not associate with membranes, indicating that it might play a unique or different role in cytoplasmic sucrose degradation from that of SUS1 or SUS-SH1. Moreover, SUS2 may function to regulate SUS1 membrane association by formation of SUS1-SUS2 hetero-oligomers, which contained essentially all of the cell’s SUS 2 protein. This study provides fundamental information about isoform specific cellular functions and the oligomerization and phosphorylation status of SUS. The results add to the foundation of knowledge required for future molecular genetic manipulation of crop plants for altered allocation of carbon in sink organs to enhance yield.

Technical Abstract: 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 Thr 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 likely impart isoform specific functional roles.