Francisella IglG protein and the DUF4280 proteins: PAAR- like proteins in non-canonical Type VI secretion systems?
Résumé
Type VI secretion systems (T6SS) are bacterial molecu-
lar machines translocating effector proteins into target
cells. T6SS are widely present in Gram-negative bacte-
ria where they predominantly act to kill neighboring
bacteria. This secretion system is reminiscent of the
tail of contractile bacteriophages and consists of a con-
tractile sheath anchored in the bacterial envelope and
an inner tube made of stacks of the Hcp protein. The
Hcp tube is capped with a VgrG trimer and a spike pro-
tein termed PAAR, which acts as the membrane-
puncturing device. Francisella tularensis, the agent of
tularemia, is an intracellular bacterium replicating
within the host cytosol. Upon entry into the host cell, F.
tularensis rapidly lyses the host vacuolar membrane to
reach the host cytosol. This escape is dependent on the
Francisella Pathogenicity Island (FPI), which is encod-
ing an atypical T6SS. Among the 17 proteins encoded
by the FPI, most of them required for virulence, eight
have some homology to canonical T6SS proteins. We
recently identified the function of one protein of un-
known function encoded within the FPI, IglG. By three-
dimensional modelling and following validation by
different techniques, we found that IglG adopts a fold
resembling the one of PAAR proteins. Importantly, IglG
features a domain of unknown function DUF4280, pre-
sent in numerous bacterial species. We thus propose
to rename this domain of unknown function, PAAR-like
domain, and discuss here the characteristics of this
domain and its distribution in both Gram-negative and
Gram-positive bacteria.
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