4.D.3.1.7
Cellulose synthase complex for secondary cell wall synthesis including CESA1, 3 and 6, all catalytic subunits (see 4.D.3.1.4) (Watanabe et al. 2015). A homolog (83% identical to Q8LPK5) from Populus tomentosa (Chinese white poplar) (CesA8) alone secretes the nascent polymer through a channel formed by its own transmembrane domain dependent on a lipid bilayer and Mn²⁺ to form microfibrils in vitro (Purushotham et al. 2016). CesA8, of 1055 aas and 6 - 8 TMSs, makes cellulose from UDP-activated glucose molecules. CesA8 is a membrane-integrated processive glycosyltransferase (Verma et al. 2023). It couples the elongation of the cellulose polymer with its translocation across the plasma membrane.  Verma et al. 2023 presented substrate and product-bound cryogenic EM structures of the homotrimeric CesA8 from hybrid aspen (poplar). UDP-glucose binds to a conserved catalytic pocket adjacent to the entrance to a transmembrane channel. The substrate's glucosyl unit is coordinated by conserved residues of the glycosyltransferase domain and amphipathic interface helices. Site-directed mutagenesis of a conserved gating loop capping the active site reveals its critical function for catalytic activity. Molecular dynamics simulations revealed prolonged interactions of the gating loop with the substrate molecule, particularly across its central conserved region. These transient interactions likely facilitate the proper positioning of the substrate molecule for glycosyl transfer and cellulose translocation. Molecular dynamics simulations support persistent gating loop - substrate interactions. The gating loop helps to position the substrate molecule to facilitate cellulose elongation (Verma et al. 2023).