Catalytic subunit of cellulose synthase terminal complexes ('rosettes'), required for beta-1,4-glucan microfibril crystallization, a major mechanism of the cell wall formation. Involved in the primary cell wall formation. [(1->4)-beta-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->4)-beta-D-glucosyl](n+1) + H(+) + UDP Binds 2 Zn(2+) ions per subunit. Glycan metabolism; plant cellulose biosynthesis. Homodimer. Interaction through zinc finger domain. Strongly and ubiquitously expressed. Localized in some dividing and expanding cells, as well as in vascular tissues. Expressed throughout the embryo during all steps of embryogenesis, and decrease toward the bent-cotyledon stage. Plants display reduced cellulose synthesis affecting microtubule orientation, general cell size, hypocotyl growth and seeds production. Partially redundant with CESA6. Belongs to the glycosyltransferase 2 family. Plant cellulose synthase subfamily.