Dimerization of cotton fiber cellulose synthase catalytic subunits occurs via oxidation of the zinc-binding domains

Abstract

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 Zn²⁺, 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 Zn²⁺ 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 325′615 (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.