Diverse specificity of cellulosome attachment to the bacterial cell surface.

Joana L A Brás,Edward A Bayer,Benedita A Pinheiro,Aldino Viegas,Victor D Alves,Maria João Romão,Shabir Najmudin,Steven Smith,Pedro Bule,Harry J Gilbert,Ana Luísa Carvalho,Holly L Spencer,Carlos M G A Fontes,Virginia M R Pires,Fiona Cuskin,Kate Cameron

doi:10.1038/srep38292

Abstract

During the course of evolution, the cellulosome, one of Nature’s most intricate multi-enzyme complexes, has been continuously fine-tuned to efficiently deconstruct recalcitrant carbohydrates. To facilitate the uptake of released sugars, anaerobic bacteria use highly ordered protein-protein interactions to recruit these nanomachines to the cell surface. Dockerin modules located within a non-catalytic macromolecular scaffold, whose primary role is to assemble cellulosomal enzymatic subunits, bind cohesin modules of cell envelope proteins, thereby anchoring the cellulosome onto the bacterial cell. Here we have elucidated the unique molecular mechanisms used by anaerobic bacteria for cellulosome cellular attachment. The structure and biochemical analysis of five cohesin-dockerin complexes revealed that cell surface dockerins contain two cohesin-binding interfaces, which can present different or identical specificities. In contrast to the current static model, we propose that dockerins utilize multivalent modes of cohesin recognition to recruit cellulosomes to the cell surface, a mechanism that maximises substrate access while facilitating complex assembly.

Highlights

Cellulosomes are highly efficient nanomachines produced by anaerobic microbes to deconstruct plant structural carbohydrates[1,2,3]
In C. thermocellum, the cellulosome is organized by primary scaffoldin ScaA and five different anchoring scaffoldins (Fig. 1)
Transcriptomic and proteomic studies revealed that this protein is upregulated when C. thermocellum is grown on recalcitrant carbohydrates such as cellulose[11,12]

Summary

Introduction

Cellulosomes are highly efficient nanomachines produced by anaerobic microbes to deconstruct plant structural carbohydrates[1,2,3]. Coh-Doc complexes exhibit one of the strongest protein-protein affinities known in Nature, and it is evident that the precise assembly and cell surface attachment of cellulosomes are orchestrated by the different specificities displayed by type-I and type-II Coh-Doc interactions, respectively[1,8]. The crystal structure of the C. thermocellum primary scaffoldin (ScaA) type-II Doc in complex with the Coh of the anchoring scaffoldin ScaF revealed striking differences with type-I Coh-Doc complexes[6]. Upstream of the UNKs is an X-module and the C-terminal type-II Doc. A. cellulolyticus cellulosome presents a similar degree of complexity recruitment of the macromolecular complex to the cell surface may involve the activity of an adaptor scaffoldin (ScaB) as well as typical anchoring scaffoldins (ScaD and ScaF) (Fig. 1). In contrast to the previously suggested “static” model, the data support the hypothesis that binding of large cellulosomal complexes to the bacterial surface is mediated by a dual-binding mode that, assumes a diversity of mechanisms in different bacteria

Methods

Results

Conclusion