J. Biol. Chem. 10.1074/jbc.M114.607192

Cellulose polymers found in plant cell walls are synthesized by cellulose synthase complexes (CESA), but the mechanism for conversion of individual polymers into microfibrils remains unclear. COBRA is coexpressed with CESA, and mutations of COBRA are linked to changes in cellulose, but its specific function is unknown. Sorek et al. now connect these aspects in their study of COBRA localization and impacts on cellulose assembly. The authors first track COBRA in elongating root cells, confirming the protein is found at the plasma membrane, as expected given its previously identified GPI anchor. COBRA was less abundant than CESA and moved through the cell more slowly, suggesting that the proteins were not directly coupled; however, COBRA was strongly depleted by a cellulose synthase inhibitor, supporting a functional link. Similarly, tracking CESA in wild-type cells and a COBRA knockout indicated that COBRA slows down but does not otherwise affect movement of CESA. In vitro, COBRA binds both individual glucan chains and crystalline cellulose but shows a preference for individual chains in a direct competition assay. Analysis of cell walls by solid state NMR showed that cellulose is decreased in knockout cells and that the remaining cellulose is less crystalline and contains a larger number of shorter glucan chains. The combined results lead the authors to propose that COBRA functions in proximity to, but not associated with, CESA to align glucan chains and create cellulose microfibrils.