Direct Visualization by Cryo-EM of the Mycobacterial Capsular Layer: A Labile Structure Containing ESX-1-Secreted Proteins

Sani, Musa; Houben, Edith N. G.; Geurtsen, Jeroen; Pierson, Jason; de Punder, Karin; van Zon, Maaike; Wever, Brigitte; Piersma, Sander R.; Jiménez, Connie R.; Daffé, Mamadou; Appelmelk, Ben J.; Bitter, Wilbert; van der Wel, Nicole; Peters, Peter J.
March 2010
PLoS Pathogens;Mar2010, Vol. 6 Issue 3, p1
Academic Journal
The cell envelope of mycobacteria, a group of Gram positive bacteria, is composed of a plasma membrane and a Gramnegative- like outer membrane containing mycolic acids. In addition, the surface of the mycobacteria is coated with an illcharacterized layer of extractable, non-covalently linked glycans, lipids and proteins, collectively known as the capsule, whose occurrence is a matter of debate. By using plunge freezing cryo-electron microscopy technique, we were able to show that pathogenic mycobacteria produce a thick capsule, only present when the cells were grown under unperturbed conditions and easily removed by mild detergents. This detergent-labile capsule layer contains arabinomannan, a-glucan and oligomannosyl-capped glycolipids. Further immunogenic and proteomic analyses revealed that Mycobacterium marinum capsule contains high amounts of proteins that are secreted via the ESX-1 pathway. Finally, cell infection experiments demonstrated the importance of the capsule for binding to cells and dampening of pro-inflammatory cytokine response. Together, these results show a direct visualization of the mycobacterial capsular layer as a labile structure that contains ESX-1-secreted proteins.


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