Fig. 4.

Structural comparisons of AtCESA3^CatD to BcsA and PttCESA8 reveal different mechanisms in coordinating UDP-Glc. (A) Structural comparison of AtCESA3^CatD (salmon) to the BcsA catalytic domain (PDB ID code 4HG6; gray). An enlarged view of the catalytic sites involving UDP-Glc and metal ion binding in AtCESA3 and BcsA is shown on the Right. The bound cellulose and UDP-Glc are shown as sticks. For cellulose, C and O atoms are colored gray and red; for UDP-Glc in BcsA, C, O, N, and P atoms are colored gray, red, blue, and orange, respectively. (B) Structural alignment of AtCESA3^CatD and full-length PttCESA8 (PDB ID code 6WLB). The PttCESA8 is colored gray, with its N-terminal region (E157-Q249) prior to the GT domain in blue. The two β-strands connecting the first two transmembrane helices to cytosolic domains are indicated by the starting and ending amino acids, respectively. A detailed view of catalytic sites and the glucan chain channel of AtCESA3^CatD and PttCESA8 is shown on the Right. The conserved residues that bind UDP-Glc are show as sticks, and the bound Mn²⁺ is shown as spheres. The cellulose chain in PttCESA8 is shown as sticks, with C and O atoms colored gray and red. The conformational change in the catalytic residue D765 in AtCESA3^CatD compared with its equivalent residue D676 in PttCESA8 is indicated by the solid arrow. A proposed change in UDP-Glc in AtCESA3^CatD, following the same change direction as that of D765, is indicated by the dashed arrow. β6 in AtCESA3^CatD and canonical QxxRW motif in both structures are labeled. Structural comparison also clearly demonstrates a large conformational change in the helix comprising the QxxRW motif, implying that it is dynamic and flexible.