Structural Studies of Plant CESA Support Eighteen CESAs in the Plant CSC

Abstract

The cellulose synthesis in plants is carried by a large multi-subunit transmembrane protein complex known as cellulose synthesis complex (CSC). The structural information on plant cellulose synthase (CESA) proteins that constitute CSC has remained challenging over years due to inherent complexities in the CSC leading to speculations and debates on the composition of CSC and the number of cellulose chains in a microfibril. Here, we report our findings on the structural properties of catalytic domain of Arabidopsis thaliana CESA1 (ATCESA1CatD) analyzed using small-angle scattering and computational modeling techniques. Our main findings include low resolution structures of ATCESA1CatD in monomeric and trimeric complex forms that provide the first experimental evidence supporting the self-assembly of CESAs into stable trimeric complexes. This is of immense importance in the context of CSC formation by plant CESAs and addresses a long standing question in plant biology - how many CESAs in the plant cellulose synthesis complex? Further, the scattering data in combination with computational modeling provided insight into the potential arrangement the monomers in the catalytic trimer and the relative arrangement of P-CR and CSR regions that are unique in the plants. Comparison of the size of the trimer complex with the dimensions of CSCs from TEM images provides compelling evidence that each lobe of a CSC contains three CESAs rather than the long-standing model of six CESAs within each lobe of a rosette CSC. To our knowledge, these studies are the first experimental evidence that CESA trimers form the lobes of rosette CSCs providing strong support for the hexamer of trimers model that synthesizes an 18-chain cellulose microfibril as the fundamental product of cellulose synthesis in plants.