| Abstract: | Cellulose microfibrils are para-crystalline arrays of several dozenlinear (1→4)-β-d-glucan chains synthesized at the surface ofthe cell membrane by large, multimeric complexes of synthase proteins. Recombinantcatalytic domains of rice (Oryza sativa) CesA8 cellulose synthaseform dimers reversibly as the fundamental scaffold units of architecture in thesynthase complex. Specificity of binding to UDP and UDP-Glc indicates a properlyfolded protein, and binding kinetics indicate that each monomer independentlysynthesizes single glucan chains of cellulose, i.e., two chains per dimer pair. Incontrast to structure modeling predictions, solution x-ray scattering studiesdemonstrate that the monomer is a two-domain, elongated structure, with the smallerdomain coupling two monomers into a dimer. The catalytic core of the monomer isaccommodated only near its center, with the plant-specific sequences occupying thesmall domain and an extension distal to the catalytic domain. This configuration isin stark contrast to the domain organization obtained in predicted structures ofplant CesA. The arrangement of the catalytic domain within the CesA monomer and dimerprovides a foundation for constructing structural models of the synthase complex anddefining the relationship between the rosette structure and the cellulosemicrofibrils they synthesize. |
