In silico structure prediction of full-length cotton cellulose synthase protein (GhCESA1) and its hierarchical complexes

Cellulose synthase (CESA) polymerizes glucose into beta-1,4-glucan chains that assemble to form cellulose microfibrils. Cellulose is the most abundant natural polymer in the world and a major structural component of the plant cell wall. An understanding of cellulose synthesis in plants is crucial for advancement in biofuels research and requires high-resolution 3D CESA structures. However, the determination of 3D structures of plant CESAs and their specific hierarchical arrangement into cellulose synthesis complexes (CSCs) has been a challenge. The prediction of CESA structures using computational methods presents a challenge due to poor sequence homology with resolved structures, long sequence, and structural complexity due to a mixture of globular, transmembrane, and intrinsically disordered regions. Herein, we present a 3D atomic-resolution model of a full-length (974-aa) cotton CESA (GhCESA1) structure using a variety of computational techniques with a reasonable ProSA-web z-score of - 8.32 (PDB available in SI). The overall fold of the CESA model indicates that there are similarities to BcsA bacterial cellulose synthase, such as the transmembrane topology and the internalization of conserved catalytic motifs. The plant-specific regions (CSR, P-CR, and N-term) fold into distinct subdomains, indicating the importance of these regions in CESA assembly into plant CSCs. We further examined possible assemblies of CESA monomers forming trimers and 18-mer CSCs, and we compared our results to those obtained by freeze fracture transmission electron microscopy. We observed that there are numerous competing ways in which CESAs may be arranged into homotrimers and CSCs. Our predicted structure can be used to probe CESA structure-activity relationships, select and subsequently test possible mutants, and investigate CESA aggregation into CSCs and microfibril formation to optimize biomass properties.