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 secondary cell wall formation. Required for the xylem cell wall thickening. [(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. Interacts with CESA4 and CESA8. Assembly with CESA4 and CESA8 is required for functional complex and localization in secondary cell wall deposition sites. Interacts with STL1 and STL2, but not with GOT1 (PubMed:27277162). Binds to TED6 (PubMed:19383897). Confined to secondary cell wall developing tissues such as xylems and interfascicular regions. Expressed in young plants, stems and flowers, but not in leaves, roots and shoots. Not found in embryos. Increasing amount as stems mature. Phosphorylated protein may be subject to proteasome degradation. S-acylated at Cys-621, Cys-623, Cys-624, Cys-626, Cys-1022 and Cys-1026. The ratio of acylation by either palmitate or stearate is unknown. S-acylation is essential for plasma membrane localization. Reduced plant size, reduced content of cellulose, collapsed xylem vessels and wilting of inflorescence stems (PubMed:9165747). Enhanced resistance to the pathogens Ralstonia solanacearum and Plectosphaerella cucumerina (PubMed:17351116). Belongs to the glycosyltransferase 2 family. Plant cellulose synthase subfamily.