Point mutations in the catalytic domain disrupt cellulose synthase (CESA6) vesicle trafficking and protein dynamics

Abstract

Cellulose, as the main component of the plant cell wall, is synthesized by a multimeric protein complex named the cellulose synthase (CESA) complex or CSC. In plant cells, CSCs are transported through the endomembrane system to the PM, but how catalytic activity or conserved motifs around the catalytic core domain influence vesicle trafficking or protein dynamics is not well understood. Here, we used a functional YFP-tagged AtCESA6 and site- directed mutagenesis to create 18 single amino acid replacement mutants in key motifs of the catalytic domain including DDG, DXD, TED and QXXRW, to comprehensively analyze how catalytic activity affects plant growth, cellulose biosynthesis, complex formation, as well as CSC dynamics and trafficking. Plant growth and cellulose content were reduced by nearly all mutations. Moreover, mutations in most conserved motifs reduced the speed of CSC movement in the PM as well as delivery of CSCs to the PM. Interestingly, the abundance of YFP-CESA6 in the Golgi apparatus was increased or reduced by mutations in DDG and QXXRW motifs, respectively. Post-Golgi trafficking of CSCs was also differentially perturbed by these mutations and, based on these phenotypes, the 18 mutations could be divided into two major groups. Group I comprises mutations causing significantly increased fluorescence intensity of YFP-CESA6 in Golgi with either an increase or no change in the abundance of cortical small CESA-containing compartments (SmaCCs). In contrast, Group II represents mutations with significantly decreased fluorescence intensity of YFP-CESA6 in Golgi and/or reduced SmaCC density. In addition, two Group II mutations in the QXXRW motif reduced CSC assembly in the Golgi. We propose that Group I mutations cause CSC trafficking defects whereas Group II mutations, especially in the QXXRW motif, affect normal CSC rosette formation in the ER or Golgi and hence interfere with subsequent CSC trafficking. Collectively, our results demonstrate that the catalytic domain of CESA6 is essential not only for cellulose biosynthesis, but also CESA complex formation, protein folding and dynamics, vesicle trafficking, or all of the above. One sentence summaryA comprehensive mutational analysis of the catalytic domain of Arabidopsis CESA6 reveals distinct roles for conserved motifs in CSC vesicle trafficking, protein complex formation, or protein dynamics