Phosphorylation of the Type VI secretion system sheath controls Francisella virulence

Many Gram-negative bacteria use type VI secretion systems (T6SS) to translocate effector proteins into eukaryotic or prokaryotic cells. Francisella possesses a non-canonical T6SS encoded on the Francisella pathogenicity island (FPI) that is essential for efficient phagosomal escape and access to the cytosol of infected host cells. Using a global and site-specific phosphoproteomic analysis of Francisella, we identified a unique phosphorylation site on tyrosine 139 of IglB, the TssC homologue and a key component of the T6SS contractile sheath. Phosphorylated IglB was exclusively found in the non-oligomerized form of the protein, suggesting that sheath assembly requires IglB dephosphorylation. All the different amino acid substitutions at site 139 of IglB all impaired sheath assembly. The data presented suggest that the phosphorylation status of IglB plays a critical role in T6SS activity and, hence, Francisella pathogenicity. We propose that this post-translational modification of the sheath may constitute a previously unrecognized mechanism to modulate the dynamics of assembly-disassembly of the T6SS.