Structures and roles of BcsD and partner scaffold proteins in proteobacterial cellulose secretion.

Cellulose is the world's most abundant biopolymer, and similar to its role as a cell wall component in plants, it is a prevalent constituent of the extracellular matrix in bacterial biofilms. Although bacterial cellulose (BC) was first described in the 19th century, it was only recently revealed that it is produced by several distinct types of Bcs secretion systems that feature multiple accessory subunits in addition to a catalytic BcsAB synthase tandem. We recently showed that crystalline cellulose secretion in the Gluconacetobacter genus (α-Proteobacteria) is driven by a supramolecular BcsH-BcsD scaffold-the "cortical belt"-which stabilizes the synthase nanoarrays through an unexpected inside-out mechanism for secretion system assembly. Interestingly, while bcsH is specific for Gluconacetobacter, bcsD homologs are widespread in Proteobacteria. Here, we examine BcsD homologs and their gene neighborhoods from several plant-colonizing ß- and γ-Proteobacteria proposed to secrete a variety of non-crystalline and/or chemically modified cellulosic polymers. We provide structural and mechanistic evidence that through different quaternary structure assemblies BcsD acts with proline-rich BcsH, BcsP, or BcsO partners across the proteobacterial clade to form synthase-interacting intracellular scaffolds that, in turn, determine the biofilm strength and architecture in species with strikingly different physiology and secreted biopolymers.

Bacterial crystalline cellulose secretion via a supramolecular BcsHD scaffold.

Cellulose, the most abundant biopolymer on Earth, is not only the predominant constituent of plants but also a key extracellular polysaccharide in the biofilms of many bacterial species. Depending on the producers, chemical modifications, and three-dimensional assemblies, bacterial cellulose (BC) can present diverse degrees of crystallinity. Highly ordered, or crystalline, cellulose presents great economical relevance due to its ever-growing number of biotechnological applications. Even if some acetic acid bacteria have long been identified as BC superproducers, the molecular mechanisms determining the secretion of crystalline versus amorphous cellulose remain largely unknown. Here, we present structural and mechanistic insights into the role of the accessory subunits BcsH (CcpAx) and BcsD (CesD) that determine crystalline BC secretion in the Gluconacetobacter lineage. We show that oligomeric BcsH drives the assembly of BcsD into a supramolecular cytoskeletal scaffold that likely stabilizes the cellulose-extruding synthase nanoarrays through an unexpected inside-out mechanism for secretion system assembly.