CesA Dimers are the Fundamental Building Blocks of a Cellulose Synthase Complex
Cellulose is a para-crystalline array of two to three dozen (1→4)-β-D-glucan polysaccharides that forms the fundamental scaffold molecule of plant cell walls and gives rise to cell shape and structural rigidity. We show here that recombinant catalytic domains of rice CesA8 cellulose synthase reversibly form dimers dependent on protein concentration and the removal of thiol reducing agents. Small-angle x-ray scattering studies predict a two-domain elongated monomer, with the smaller domain functioning to couple two monomers into a dimer. Structure modeling of the catalytic domain truncated to remove two large plant-specific sequences not found in bacterial synthases gives close alignment with the Rhodobacter sphaeroides cellulose synthase catalytic domain (BcsA). Modeling against 18 additional targets gave closest similarity to an E. coli chondroitin polymerase when the plant-conserved region (P-CR) and the class-specific region (CSR) are included, and gives a molecular shape consistent with the findings from small-angle x-ray scattering studies. However, ChondP distorts the active site, separating the essential conserved motifs of substrate binding and glycosyl transfer. A composite model comprising complementary domains of similarity to BcsA and a Zea mays oxidoreductase gives a structure that preserves the catalytic domain and places the P-CR and CSR regions in a position where they can interact and stabilize dimers. Independent sites of glycosyl transfer on each monomer and production of two (1→4)-β-D-glucan chains per dimer account for a size consistent with known synthase complex dimensions.
Researchers should cite this work as follows:
- Olek AT, Rayon C, Makowski L, Kim HR, Ciesielski P, Paul LN, Ghosh S, Kihara D, Crowley M, Himmel M, Bolin J, Carpita NC. CesA dimers are the fundamental building blocks of a cellulose synthase complex. American Society of Plant Biologists. Providence, RI. July 20-24.