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Bacterial gliding motility is mediated by apicomplexan-like cleavage of a surface adhesin

Islam, Salim Timo . Bacterial gliding motility is mediated by apicomplexan-like cleavage of a surface adhesin In: 66th Annual Conference of the Canadian Society of Microbiologists, 15 juin 2016, Toronto.

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Résumé

Single Myxococcus xanthus cells are capable of “gliding” over solid surfaces in the absence of flagella and type IV pili (T4P); this process involves coordination between several protein modules in focal adhesion complexes (FACs) that span the cell envelope. However, the mechanism by which the Glt gliding machinery engages with the underlying substratum remains unknown. The CglB protein is an ideal candidate for this role as (i) it has been identified in the outer membrane (OM), (ii) it can be transferred between bacteria via cell-cell contact, and (iii) is essential for gliding.

Fold recognition, 3D structure modelling, and evolutionary couplings analyses revealed that CglB was structurally similar to known microbial tip pilin surface adhesins. Site-directed mutagenesis of an identified metal ion-dependent adhesion site (MIDAS) motif and motility complementation studies further supported the functional homology with adhesins. Tracking of fluorescent FACs in ΔcglB cells revealed continued trafficking of intact Glt motility complexes inside the cell, despite a complete lack of single-cell motility.

To directly examine motor–surface coupling, polystyrene beads were placed on wild-type and ΔcglB cell surfaces via optical trap, followed by particle-tracking analyses of bead translocation events, which were severely compromised in the latter strain. Finally, α-CglB immunofluorescence labelling of gliding M. xanthus cells detected CglB at stationary FACs, with foci remaining substratum-fixed even after cell passage. These data indicate that CglB at FACs is responsible for coupling the trafficked motility complexes inside the cell to the substratum to power single-cell propulsion.

Intriguingly, CglB was found to be depleted in cells lacking all OM gliding machine components; however, the corresponding supernatants in these mutants were enriched for CglB. Supernatant CglB was smaller than the variant from intact cells, suggesting that proteolytic cleavage was responsible for its release from the cell. Inhibition of metalloprotease activity via ion chelation restored detectable amounts of CglB in non-retaining mutant backgrounds, supporting metalloprotease-dependent liberation of CglB from intact cells.

These results are remarkably similar to the Plasmodium (malaria parasite) mechanism of gliding motility in which a substratum-adhered TRAP adhesin protein, linked to the microbial cytoskeleton, is translocated towards the rear of the cell and must be cleaved from the tip of a surface-bound adhesion point before a new adhesion can be formed at the leading pole. These findings also have implications for T4P retraction and subsequent re-extension as they provide evidence of tip pilin proteolysis (as opposed to recycling) in a bacterium.

Type de document: Document issu d'une conférence ou d'un atelier
Mots-clés libres: -
Centre: Centre INRS-Institut Armand Frappier
Date de dépôt: 26 déc. 2017 18:38
Dernière modification: 26 déc. 2017 18:38
URI: http://espace.inrs.ca/id/eprint/5733

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