Direct visualization by cryo-EM of the mycobacterial capsular layer: a labile structure containing ESX-1-secreted proteins.

M. Sani, E.N.G. Houben, J.J.G. Geurtsen, J. Pierson, K. de Punder, M. van Zon, B. Wever, S.R. Piersma, C.R. Jimenez, M. Daffe, B.J. Appelmelk, W. Bitter, N. Wel, P.J. Peters

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The cell envelope of mycobacteria, a group of Gram positive bacteria, is composed of a plasma membrane and a Gram-negative-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, α-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. © 2010 Sani et al.
Original languageEnglish
Pages (from-to)e1000794
Number of pages10
JournalPLoS pathogens
Issue number3
Publication statusPublished - 2010

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