Rice Cellulose SynthaseA8 Plant-Conserved Region Is a Coiled-Coil at the Catalytic Core Entrance

Abstract

The crystallographic structure of a rice (Oryza sativa) cellulose synthase, OsCesA8, plant-conserved region (P-CR), one of two unique domains in the catalytic domain of plant CesAs, was solved to 2.4 Å resolution. Two antiparallel a-helices form a coiledcoil domain linked by a large extended connector loop containing a conserved trio of aromatic residues. The P-CR structure was fit into a molecular envelope for the P-CR domain derived from small-angle X-ray scattering data. The P-CR structure and molecular envelope, combined with a homology-based chain trace of the CesA8 catalytic core, were modeled into a previously determined CesA8 small-angle X-ray scattering molecular envelope to produce a detailed topological model of the CesA8 catalytic domain. The predicted position for the P-CR domain from the molecular docking models places the P-CR connector loop into a hydrophobic pocket of the catalytic core, with the coiled-coil aligned near the entrance of the substrate UDP-glucose into the active site. In this configuration, the P-CR coiled-coil alone is unlikely to regulate substrate access to the active site, but it could interact with other domains of CesA, accessory proteins, or other CesA catalytic domains to control substrate delivery.

Cellulose synthase ‘class specific regions’ are intrinsically disordered and functionally undifferentiated

“…Our docked model (Figure A) is similar to prior structural models based on small angle X‐ray scattering data from a homotrimer of recombinant AtCESA1 cytosolic domains (Vandavasi et al ) and an in silico homotrimer of a nearly complete GhCESA1 protein (Nixon et al ). A third trimer structure assembled from the SAXS molecular envelopes of native recombinant monomeric CesA8 catalytic domains (Olek et al ), with P‐CR contacts that conform to a three‐fold contact within the crystal structure, is not possible when the membrane spanning domains are added (Rushton et al ). CESA catalytic subunits have also been shown to dimerize (Olek et al ).…”

“…The 126 amino acid P‐CRs are conserved both in sequence and structure across diverse CESA isoforms (Pear et al ; Sethaphong et al ; Rushton et al ). In contrast, the CSRs have variable lengths (e.g., 86‐106 amino acids in Arabidopsis ) and poorly conserved sequences (Pear et al ; Vergara and Carpita ; Carroll and Specht ; Kumar et al ).…”

“…Experimental modeling of CESA homotrimers has shown different possibilities, with CSRs either forming CESA‐CESA contacts, within lobes, or residing on the periphery of lobes where they could form CESA‐CESA contacts between lobes or interface with other partner proteins (Sethaphong et al ; Nixon et al ; Sethaphong et al ; Vandavasi et al ; Rushton et al ). However, none of these models are consistent with all available data (Vandavasi et al ; Rushton et al ).…”

Secondary cell wall biosynthesis

“…One of these mutants, fra5, carrying a missense mutation in the catalytic domain of CESA7, exhibits a semidominant cellulose-deficient phenotype, indicating that the mutant protein is probably incorporated into the rosette complexes and inhibits the activities of other CESA subunits (Zhong et al, 2003). The P-CR and the CSR were proposed to be involved in protein-protein interactions between CESAs or between CESAs and their interacting proteins but their exact functions await further investigation (Slabaugh et al, 2014;Rushton et al, 2017).…”

Cellulose synthase complex organization and cellulose microfibril structure

“…The protein backbone of the P-CR forms an elongated loop with two large helices contacting one another. Docking of this structure onto the structure of the catalytic domain derived from SAXS finds that the best-fit models place the base of the P-CR near the opening to the catalytic site [33], a position that is not easy to reconcile with that proposed for the AtCESA1Cat trimer [32]. These apparent differences in structures predicted from rice CESA8 and Arabidopsis CESA1 may reflect a real difference in the structure of the central catalytic domain between these two different CESA proteins.…”