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Title: THE THIRD SUCROSE SYNTHASE IN <1>ZEA MAYS</1>(SUS3) IS A UBIQUITOUS PROTEIN THAT IS PHOSPHORYLATED AND MEMBRANE ASSOCIATED.

Author
item DUNCAN, KATERI - UNIVERSITY OF ILLINOIS
item HARDIN, SHANE - UNIVERSITY OF ILLINOIS
item Huber, Steven

Submitted to: Plant Biology Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 4/7/2005
Publication Date: 5/1/2005
Citation: Duncan, K.A., Hardin, S.C., Huber, S.C. The third sucrose synthase in <1>zea mays</1>(sus3) is a ubiquitous protein that is phosphorylated and membrane associated [abstract]. Plant Biology Annual Meeting. Available: http://abstracts.aspb.org/pb2005/public/P56/7486.html.

Interpretive Summary:

Technical Abstract: Sucrose synthase (SUS) is a vital enzyme in plant metabolism as it serves to cleave sucrose into UDP-glucose and fructose. <1>Zea mays has three known sucrose synthase isoforms: SUS1, SH1, and SUS3. The SUS3 isoform was only recently characterized at the transcript level. Recent evidence using a peptide antibody that specifically recognizes the SUS3 isoform suggests that it is a ubiquitous protein. Abundant SUS3 protein is found in developing kernals, leaf midvein, internode cortex tissue, and etiolated roots; its presence in midveins suggests that SUS3 might function in energizing sucrose transport. Results using both in vivo and in vitro approaches suggest that SUS3 is partially associated with microsomes prepared from developing kernels, and the elongation zone of developing maize leaves, and thus could play a role in cellulose biosynthesis in plants. Recombinant SUS3, as well as an N-terminal synthetic SUS3 peptide (KLDRNPSIRDR), can be phosphorylated in vitro by a calcium dependent protein kinase (CDPK) as detected by 32P-incorporation and the phospho-specific ProQ Diamond stain. Together, these results suggest that SUS3 may be phosphorylated at Ser11, similar to the phosphorylation of SUS1 at Ser15. Whether the Ser11 site is SUS3 is involved in membrane association like that of Ser15 in SUS1 is currently under investigation. In contrast, the isoforms may differ in phosphorylation at a C-terminal site, corresponding to Thr786 in SUS1. The sequence surrounding this residue is highly, but not strictly, conserved in the three isoforms. In vivo phosphorylation of midvein SUS at Thr786 was established by a phospho-specific SUS antibody, however, it is not clear which isoform(s) is involved. Peptide kinase assays using protein fractions purified from midvein tissue indicate that there are both CDPK and SnRK1-like (Snf1-Related Kinase) kinases present in midveins (using peptides KLTRLHSLRERLG in presence of calcium and KGRMRRISSVEMMK in the absence of calcium). The presence of these kinases in midveins suggests that they may play a role in the phosphorylation of SUS3. The occurrence and possible regulatory significance of C-terminal phosphorylation of SUS isoforms is currently under investigation.