Inner membrane protein OutB is covalently attached to peptidoglycan in the γ-proteobacterium Dickeya dadantii

Gram-negative bacteria possess a multilayered envelope comprising an inner membrane (IM), a thin peptidoglycan (PG) layer, and an outer membrane (OM). In Escherichia coli and certain other γ-proteobacteria, Braun lipoprotein (Lpp) covalently tethers the OM to PG. Only a few other OM proteins have been found to be covalently linked to PG in Gram-negative bacteria. Here, we showed that in the phytopathogenic γ-proteobacterium Dickeya dadantii , an IM protein, OutB, is covalently attached to PG, thereby tethering itself and the associated type 2 secretion system to the cell wall. In contrast to Lpp, OutB reaches the PG layer from the IM side. By modifying the length of Lpp, which would displace the PG layer in the periplasm, we found that the elongated Lpp+21 improved OutB attachment to PG, whereas the shortened LppΔ21 reduced it. We showed that two L,D-transpeptidases, Ldt03 and Ldt84, tether Lpp and OutB to PG by the same catalytic mechanism involving the formation of an amide bond between their C-terminal lysine and the stem peptide. Ldt03 and Ldt84, each display substrate specificity for the type of peptide stem and preferentially cross-link Lpp to monomeric and dimeric muropeptide, respectively. The C-terminal Lpp-like box of OutB is almost identical to that of Lpp; it tolerates substantial amino acid substitutions and allows PG-tethering of a bona fide periplasmic protein. Thus, it seems possible that the repertoire of periplasmic and membrane proteins tethered to PG may be more extensive than currently assumed.