IN DICKEYA DADANTI, THE TYPE II SECRETION SYSTEM (T2SS) IS COVALENTLY ATTACHED TO THE BACTERIAL CELL WALL
Résumé
Dickeya dadantii is a broad-spectrum phytopathogenic γ-proteobacterium that provokes soft
rot disease in various plants. D. dadantii pathogenicity is caused by an arsenal of plant cellwall
degrading enzymes secreted by the Type II Secretion System (T2SS) named Out. The
Out system is a multiprotein complex spanning the cell envelope. Recently, we showed that
OutB, an inner membrane T2SS component plays an important scaffolding role and that
deletion of outB causes a striking reduction of pectinase secretion and virulence of D.
dadantii. Here, we show that in D. dadantii, OutB is covalently attached to the PG. We
investigated the molecular mechanisms of OutB attachment to peptidoglycan (PG) and the
possible implication of this phenomenon in the virulence of D. dadantii. First, the covalent
attachment of OutB was demonstrated after extraction, purification and analysis of the PG of
D. dadantii by Western blot. Second, we characterized the enzymes implicated and identified
the region and residues of OutB important for covalent attachment to PG: the two C-terminal
lysines of OutB are involved in its attachment to PG and two L,D-transpeptidases, Ldt03 and
Ldt84, catalyze this process. These results suggest that the covalent attachment of OutB to
PG allows a better stabilization of the T2SS in the bacterial cell envelope. Moreover, it shows
that the phenomenon of covalent attachment of proteins to PG in proteobacteria could be
more widespread and remains largely unexplored.