Catalytic subunit of cellulose synthase. It polymerizes uridine 5'-diphosphate glucose to cellulose, which is produced as an extracellular component for mechanical and chemical protection (By similarity). [(1->4)-beta-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->4)-beta-D-glucosyl](n+1) + H(+) + UDP Activated by bis-(3'-5') cyclic diguanylic acid (c-di-GMP). Glycan metabolism; bacterial cellulose biosynthesis. There are two conserved domains in the globular part of the protein: the N-terminal domain (domain A) contains the conserved DXD motif and is possibly involved in catalysis and substrate binding. The C-terminal domain (domain B) contains the QXXRW motif and is present only in processive glycosyl transferases. It could be involved in the processivity function of the enzyme, possibly required for holding the growing glycan chain in the active site. Belongs to the glycosyltransferase 2 family.