Molecular insights into phosphoethanolamine cellulose formation and secretion

Abstract

Phosphoethanolamine (pEtN) cellulose is a naturally occurring modified cellulose produced by several Enterobacteriaceae. The minimal components of the E. coli cellulose synthase complex include the catalytically active BcsA enzyme, an associated periplasmic semicircle of hexameric BcsB, as well as the outer membrane (OM)-integrated BcsC subunit containing periplasmic tetratricopeptide repeats (TPR). Additional subunits include BcsG, a membrane-anchored periplasmic pEtN transferase associated with BcsA, and BcsZ, a conserved periplasmic cellulase of unknown biological function. While events underlying the synthesis and translocation of cellulose by BcsA are well described, little is known about its pEtN modification and translocation across the cell envelope. We show that the N-terminal cytosolic domain of BcsA positions three copies of BcsG near the nascent cellulose polymer. Further, the terminal subunit of the BcsB semicircle tethers the N-terminus of a single BcsC protein to establish a trans-envelope secretion system. BcsC TPR motifs bind a putative cello-oligosaccharide near the entrance to its OM pore. Additionally, we show that only the hydrolytic activity of BcsZ but not the subunit itself is necessary for cellulose secretion, suggesting a secretion mechanism based on enzymatic removal of mislocalized cellulose. Lastly, we introduce pEtN modification of cellulose in orthogonal cellulose biosynthetic systems by protein engineering.

Structural basis for synthase activation and cellulose modification in theE. coliType II Bcs secretion system

“…4a-c). Contrary to the crystallized states where BcsR NTD adopts a β-hairpin conformation, here residues D 21 -S 30 fold into an N-proximal α−helix (αN) that U-turns into an extended linker before adopting the V-shaped C-terminal domain onto the BcsQ dimer interface (Fig. 4c).…”

“…2e). Importantly, while this manuscript was under preparation a separate preprint reported independently the recruitment of trimeric BcsG via BcsA NTD , based on lower-resolution cryo-EM data, subcomplex purification and AlphaFold modeling 30 . Together, these results further validate the experimental structural data presented here and the two studies integrate and redress the structure-function model of pEtN-transferase association and function.…”

Structural basis for synthase activation and cellulose modification in the E. coli Type II Bcs secretion system

“…2e). Importantly, while this manuscript was under preparation a separate preprint reported independently the recruitment of trimeric BcsG via BcsA NTD , based on lower-resolution cryo-EM data, subcomplex purification and AlphaFold modeling 30 . Together, these results further validate the experimental structural data presented here and the two studies integrate and redress the structure-function model of pEtNtransferase association and function.…”