Cellulose is a major component of the plant cell wall and plays vital roles in various physiological processes in plants, such as morphogenesis and immunity. The biosynthesis of cellulose is catalysed by a family of transmembrane glycosyltransferases called cellulose synthases (CesAs), which assemble into large cellulose synthase complexes (CSCs) that consist of different isoforms of CesAs corresponding to cellulose production and deposition in different plant developmental stages. One important approach to understand the process of cellulose biosynthesis in plants is through decoding the structural details of CesAs. The protein structures of bacterial cellulose synthesis enzymes have been reported, but the plant counterparts are largely different and remain structurally enigmatic. Recently, Jochen Zimmer’s group from the University of Virginia, USA, determined the structure of a poplar CesA (PttCesA8) in a homotrimeric complex associated with cellulose glucan chains using cryoelectron microscopy (cryo-EM).
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PttCesA8 is one of the three poplar CesAs (PttCesA4,7,8) mainly responsible for cellulose synthesis in the secondary cell wall. The researchers in Zimmer’s group achieved a technical breakthrough of CesA protein preparation, by using insect cells to express recombinant PttCesA8 proteins, and optimized a purification protocol that retains the catalytic activity of the enzyme in vitro. The obtained PttCesA8 proteins consisted of homotrimeric assemblies next to a large number of monomeric species, likely resulting from the dissociation of the sensitive complexes. To investigate the potential mechanism of CSC assembly, they decided to focus on the large trimeric particles. The cryo-EM structure of PttCesA8 trimer was determined at 3.5 Å resolution. Each PttCesA8 protein in the trimer showed a channel accommodating a short glucan chain. The cytosolic plant conserved region (PCR) in the central domain of PttCesA8 together with the associated transmembrane region is involved in generating the interphases of trimerization. The variable N-terminal domain (NTD) forms a stalk that can interact with cellulose synthase interactive (CSI) proteins (for example, CSI1) and is potentially involved in tethering CSCs to the cortical microtubules.
As proposed in this work, there are presumably six heterogeneous CesA trimers in a single plant CSC; therefore, the resolved homotrimer is not yet a good representative of CSCs in vivo. However, the authors propose that CesA trimerization likely organizes the secreted glucan chains into protofibrils, which may form the repeat unit of larger cellulose microfibrils synthesized by a CSC. This study is an important step towards understanding CSCs and cellulose synthesis in plants.
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Lei, L. Cellulose synthase trimers.
Nat. Plants (2020). https://doi.org/10.1038/s41477-020-00752-6