Crystal structure of the flexible tandem repeat domain of bacterial cellulose synthesis subunit C

Shingo Nojima,Ayumi Fujishima,Kayoko Ohuchi,Nobutaka Shimizu,Min Yao,Kenji Tajima,Koji Kato,Kento Yonezawa

doi:10.1038/s41598-017-12530-0

Shingo Nojima, Ayumi Fujishima + Show 6 more

Open Access

https://doi.org/10.1038/s41598-017-12530-0

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Abstract

Bacterial cellulose (BC) is synthesized and exported through the cell membrane via a large protein complex (terminal complex) that consists of three or four subunits. BcsC is a little-studied subunit considered to export BC to the extracellular matrix. It is predicted to have two domains: a tetratrico peptide repeat (TPR) domain and a β-barrelled outer membrane domain. Here we report the crystal structure of the N-terminal part of BcsC-TPR domain (Asp24–Arg272) derived from Enterobacter CJF-002. Unlike most TPR-containing proteins which have continuous TPR motifs, this structure has an extra α-helix between two clusters of TPR motifs. Five independent molecules in the crystal had three different conformations that varied at the hinge of the inserted α-helix. Such structural feature indicates that the inserted α-helix confers flexibility to the chain and changes the direction of the TPR super-helix, which was also suggested by structural analysis of BcsC-TPR (Asp24–Leu664) in solution by size exclusion chromatography-small-angle X-ray scattering. The flexibility at the α-helical hinge may play important role for exporting glucan chains.

Highlights

Cellulose is the most abundant biopolymer on Earth and is widely used, for example, in paper, T-shirts, and wooden furniture[1]
Full length of BcsC-tetratrico peptide repeat (TPR) domain (Asp24–Arg784) of Enterobacter CJF-002 were expressed by E. coli BL21 (DE3)
The N-terminal sequence and the molecular weight suggested that these two fragments were residues Asp24–Arg[272] and Glu295–Arg[695] of BcsC-TPR domain

Summary

Introduction

Cellulose is the most abundant biopolymer on Earth and is widely used, for example, in paper, T-shirts, and wooden furniture[1]. In Gluconacetobacter xylinum, the cellulose synthase operon codes three or four proteins: BcsA, BcsB (or BcsAB), BcsC, and BcsD14–17. AlgK and PgaA, which are located at the outer membrane and are essential proteins for exporting polysaccharides, alginate, and poly-β-1,6-GlcNAc, respectively, contain a TPR domain[23,24,25] and are classified as the same family as BcsC. An envelope model calculation showed a helical elongated extension of the structure of BcsC-TPR(N6), suggesting that the inserted α-helix confers flexibility to changes in the direction of the TPR super-helix. Such flexibility is possibly important for its cellulose exporting function

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