Modern techniques of gene cloning have identified the CesA genes as encoding the probable catalytic subunits of the plant CelS, the cellulose synthase enzyme complex visualized in the plasma membrane as rosettes. At least 10 CesA isoforms exist in Arabidopsis and have been shown by mutant analyses to play distinct role/s in the cellulose synthesis process. Functional specialization within this family includes differences in gene expression, regulation and, possibly, catalytic function. Current data points towards some CesA isoforms potentially being responsible for initiation or elongation of the recently identified sterol beta-glucoside primer within different cell types, e.g. those undergoing either primary or secondary wall cellulose synthesis. Different CesA isoforms may also play distinct roles within the rosette, and there is some circumstantial evidence that CesA genes may encode the catalytic subunit of the mixed linkage glucan synthase or callose synthase. Various other proteins such as the Korrigan endocellulase, sucrose synthase, cytoskeletal components, Rac13, redox proteins and a lipid transfer protein have been implicated to be involved in synthesizing cellulose but, apart from CesAs, only Korrigan has been definitively linked with cellulose synthesis. These proteins should prove valuable in identifying additional CelS components.

Cellulose synthase (CesA) proteins are components of CesA complexes (rosettes) and are thought to catalyze the chain elongation step in glucan polymerization. Little is understood about rosette assembly, including how CesAs interact with each other or with other components within the complexes. The first conserved region at the N terminus of plant CesA proteins contains two putative zinc fingers that show high homology to the RING-finger motif. We show that this domain in GhCesA1 can bind two atoms of Zn 2؉ , as predicted by its structure. Analysis in the yeast two-hybrid system indicates that the N-terminal portions of cotton fiber GhCesA1 and GhCesA2 containing these domains can interact to form homo-or heterodimers. Although Zn 2؉ binding occurs only when the protein is in the reduced form, biochemical analyses show that under oxidative conditions, the GhCesA1 zinc-finger domain and also the full-length protein dimerize via intermolecular disulfide bonds, indicating CesA dimerization can be regulated by redox state. We also provide evidence that the herbicide CGA 325615 (Syngenta, Basel), which inhibits synthesis of crystalline cellulose and leads to a disruption of rosette architecture, may affect the oxidative state of the zinc-finger domain that is necessary for rosette stability. Taken together, these results support a model in which at least part of the process of rosette assembly and function may involve oxidative dimerization between CesA subunits.