Structure of bacterial cellulose synthase subunit D octamer with four inner passageways

Abstract

The cellulose synthesizing terminal complex consisting of subunits A, B, C, and D in Acetobacter xylinum spans the outer and inner cell membranes to synthesize and extrude glucan chains, which are assembled into subelementary fibrils and further into a ribbon. We determined the structures of subunit D (AxCeSD/AxBcsD) with both N- and C-terminal His 6 tags, and in complex with cellopentaose. The structure of AxCeSD shows an exquisite cylinder shape (height: ∼65 Å , outer diameter: ∼90 Å , and inner diameter: ∼25 Å ) with a right-hand twisted dimer interface on the cylinder wall, formed by octamer as a functional unit. All N termini of the octamer are positioned inside the AxCeSD cylinder and create four passageways. The location of cellopentaoses in the complex structure suggests that four glucan chains are extruded individually through their own passageway along the dimer interface in a twisted manner. The complex structure also shows that the N-terminal loop, especially residue Lys6, seems to be important for cellulose production, as confirmed by in vivo assay using mutant cells with axcesD gene disruption and N-terminus truncation. Taking all results together, a model of the bacterial terminal complex is discussed.

Cellulose microfibril crystallinity is reduced by mutating C-terminal transmembrane region residues CESA1 A903V and CESA3 T942I of cellulose synthase

“…The mutated residues may be necessary to organize the individual glucan chains into the composite microfibril and thus promote crystallinity (H-bonding) by facilitating association with other glucan chains. This model is consistent with the only reported crystal structure of a CSC, an octameric structure of the periplasmic AxCeSD subunit in the CSC of the bacterium A. xylinum in complex with a 5-glucose-cellulose product (37). Here, a dimer interface was necessary to facilitate passage and proper alignment of glucan chains into a crystalline microfibril.…”

Formation of Highly Twisted Ribbons in a Carboxymethylcellulase Gene-Disrupted Strain of a Cellulose-Producing Bacterium

“…Wild-type bcsD is a homolog of acsD of G. xylinus ATCC 53582 (11,23). Cellulose productivity in the N-terminal deletion mutant of bcsD also decreased, although cellulose crystallization produced by the mutant was not investigated (39). These observations suggested that CMCax worked together with BcsD in cellulose crystallization in the wild-type strain.…”

“…The structure of BcsD (AxCeSD), a component of the cellulose synthase complex, was found to be a homo-octamer comprised of four dimer subunits with a central pore and cylindrical structure (39). The interfaces between the dimers form four spiral interstices on the wall of the molecular cylinder at a 50°angle from the cylinder (vertical) axis (39).…”

Production of Bacterial Cellulose by Acetobacter xylinum through Utilizing Acetic Acid Hydrolysate of Bagasse as Low-cost Carbon Source

“…(Iguchi et al 2000;Shah and Brown 2005;Klemm et al 2006;Hu et al 2008). Nevertheless, the high economic cost of culture media as well as low-yield production limit wide scale industrial production and extended applications of BC (Hu et al 2010;Hong et al 2011). Hence, it is challenging and meaningful to develop new approaches to high-yield BC production at the lowest cost possible.…”