Fig. 1: State-of-the-art and here-in presented structures of the Bcs macrocomplex from the E. coli Type II cellulose secretion system.

a Left, E. coli bcs operon organization, BcsA domain architecture and thumbnail representation of the secretion system topology in the E. coli envelope. Middle and right, current structural insights into complex assembly from X-ray crystallographic and electron microscopy structures^17,20,21,22. Adapted with modifications from Krasteva 2024⁴ under the CC BY 4.0 license (https://creativecommons.org/licenses/by/4.0/legalcode). NTD N-terminal domain (green), TMD transmembrane domain (wheat), GT glycosyl transferase domain (light green), PilZ c-di-GMP-sensing PilZ domain (dark red), CT C-terminal tail with amphipathic helices (orange), OM outer membrane, PG peptidoglycan, IM inner membrane, ATP adenosine triphosphate, c-di-GMP cyclic diguanylate, pEtN phosphoethanolamine, NTPase* (light gray) catalytically incompetent nucleoside triphosphatase domain, REC* (orange) phosphorylation-incompetent receiver domain, GGDEF* (dark red) degenerate diguanylate cyclase domain. Multidomain BcsB hexamerizes to form a periplasmic crown shown in two different views. The carbohydrate-binding domains are shown in shades of light purple, the flavodoxin-like domains in blue and pink, and the C-terminal tail-anchor (TA) in dark purple. Densities for BcsA^NTD (green), BcsG (light blue), BcsE (tricolor) and BcsF (dark blue) have remained practically unresolved in the macrocomplex and are represented as thumbnails, whereas crystallographic snapshots have captured two different conformations of BcsE, shown on the right^20,21. The relative REC* domain displacement and rotation are indicated (45 Å and 144 degrees, respectively). The formation of a composite c-di-GMP binding site by RxxD (arginine-two residues-aspartate) motifs from both the degenerate REC* and GGDEF* domains increases the affinity for dimeric c-di-GMP from the low micromolar to nanomolar range²¹ (bottom right). b Cartoon representations of the here-in-resolved cryo-EM structure of the assembled, c-di-GMP-saturated Bcs macrocomplex in five different views. c Cartoon representations of the here-in-resolved cryo-EM structure of the assembled Bcs macrocomplex featuring a c-di-GMP-free BcsA.