Bacillus cereus PelA_DA is a polysaccharide de-N-acetylase required for pel-dependent biofilm formation

Exopolysaccharides are key matrix determinants that provide structural integrity and regulate biomechanical properties of microbial biofilms. Biofilm exopolysaccharides often undergo modifications that determine their functional properties and localization. In Bacillus cereus ATCC 10987, PelA_DA expressed from the pelDEA_DAFG operon is a putative deacetylase required for Pel-dependent biofilm formation. To understand the molecular basis of Pel deacetylation in B. cereus ATCC 10987, we determined the crystal structure of PelA_DA to 2.51 Å. PelA_DA adopts a distinct three-domain arrangement. We demonstrate in vitro that PelA_DA deacetylates α-1,4-linked GalNAc substrates in a length-dependent manner and that the N-terminal domain functions as a carbohydrate-binding module (CBM) capable of binding both GalNAc and partially deacetylated oligosaccharides. We found that the CBM domain together with the carbohydrate esterase (CE) domain forms an elongated carbohydrate binding cleft and that each domain is the founding member of two new CAZy families, CBM114 and CE26, respectively. Further, in vivo mutagenesis demonstrated that the catalytic activity of PelA_DA is required for Pel biosynthesis in B. cereus ATCC 10987. Employing AlphaFold, we propose a model wherein the N-terminal transmembrane helix of PelA_DA interacts with PelG. This interaction positions the protein to accept the polymer for deacetylation as it emerges from the cytoplasmic membrane. The work presented herein offers insight into the role of PelA_DA in Pel biosynthesis and modification in B. cereus ATCC 10987.