Submitted to: Plant Cell Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/8/2007
Publication Date: 11/2/2007
Citation: Duncan, K.A., Huber, S.C. 2007. Sucrose synthase oligomerization and F-actin association are regulated by sucrose concentration and phosphorylation. Plant Cell Physiology. 48:1612-1623. Interpretive Summary: Sucrose synthase (SUS) is an important enzyme of sucrose metabolism in growing plant organs, such as developing seeds, and channels carbon from sucrose into various metabolic and biosynthetic pathways. The activity of SUS in a growing organ is thought to be a marker for, or possible determinant of, growth. Interestingly, this soluble protein is also found to be partially associated with cellular membranes, such as the plasma membrane, and the actin cytoskeleton. Because of its importance to plant growth, there is interest in identifying factors that control the activity and also the intracellular localization of this enzyme. The main observation of the present study is that the SUS protein can exist either as a tetramer (four subunits bound together) or dimer (two subunits bound together), and that sucrose promotes formation of the tetramer. Sucrose also promotes binding of SUS to both membranes and filamentous actin in vitro, and this can also be demonstrated in vivo by depleting plant sugars with periods of darkness. In addition, we found that the maize SUS1 isoform was predicted to contain a coiled-coil region, which are short stretches of amino acids that can self associate and can play a role in binding of subunits to form dimers or tetramers. The predicted coiled coil in the SUS1 protein has this ability but is shown not to play a role in subunit interactions. These results strongly suggest that the ability of SUS to bind to cellular structures, including membranes and the actin cytoskeleton, is regulated by the concentration of sucrose present in plant cells. The basis for this may be the formation of the tetrameric form of the protein. This may be important because it has been speculated that SUS associated with cellular structures may provide metabolites to pathways not served by the enzyme that is soluble in the cytoplasm.
Technical Abstract: Sucrose synthase (SUS) is a key enzyme in plant metabolism, as it serves to cleave the photosynthetic end product sucrose into UDP-glucose and fructose. SUS is generally assumed to be a tetrameric protein, but results in the present study suggest that SUS can form dimers as well as tetramers and that sucrose may be a regulatory factor for the oligomerization status of SUS. The oligomerization of SUS may also affect the cellular localization of the protein. We show that sucrose concentration modulates the ability of SUS1 to associate with F-actin in vitro and that CDPK-mediated phosphorylation of recombinant SUS1 at the Ser-15 site is a negative regulator of its association with actin. Although high sucrose concentrations and hyperphosphorylation have been shown to promote SUS association with the plasma membrane (Hardin et al. 2006, Plant Physiol. 141:1106-1119), we show that the opposite is true for the SUS: actin association. We also show that SUS1 has a unique 21 residue coiled-coil domain that does not appear to play a role in oligomerization, but may prove to be significant in the future for various protein-protein interactions. Collectively, these results show that SUS localization and association with cellular structures is multifaceted.