Open AccessMycobacterium tuberculosisChaperonin 10 Is Secreted in the Macrophage Phagosome: Is Secretion Due to Dissociation and Adoption of a Partially Helical Structure at the Membrane?
Author(s) -
Gianluca Fossati,
Gaetano Izzo,
Emanuele Rizzi,
Emanuela Gancia,
Daniela Modena,
Maria Luisa Moras,
Neri Niccolai,
Elena Giannozzi,
Ottavia Spiga,
L. Bono,
Piero Marone,
Biagio Eugenio Leone,
Federica Mangili,
Stephen E. Harding,
Neil Errington,
Chris Walters,
Brian E. Henderson,
Michael M. Roberts,
Anthony R. M. Coates,
Bruno Casetta,
Paolo Mascagni
Publication year - 2003
Publication title -
journal of bacteriology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.652
H-Index - 246
eISSN - 1067-8832
pISSN - 0021-9193
DOI - 10.1128/jb.185.14.4256-4267.2003
Subject(s) - chaperonin , biology , groes , circular dichroism , biophysics , secretion , mycobacterium tuberculosis , cytosol , monomer , phagosome , biochemistry , micelle , escherichia coli , crystallography , protein folding , groel , chemistry , intracellular , enzyme , polymer , organic chemistry , medicine , tuberculosis , pathology , aqueous solution , gene
To confirm that Mycobacterium tuberculosis chaperonin 10 (Cpn10) is secreted outside the live bacillus, infected macrophages were examined by electron microscopy. This revealed that the mycobacterial protein accumulates both in the wall of the bacterium and in the matrix of the phagosomes in which ingested mycobacteria survive within infected macrophages. To understand the structural implications underlying this secretion, a structural study of M. tuberculosis Cpn10 was performed under conditions that are generally believed to mimic the membrane environment. It was found that in buffer-organic solvent mixtures, the mycobacterial protein forms two main species, namely, a partially helical monomer that prevails in dilute solutions at room temperature and a dimer that folds into a beta-sheet-dominated structure and prevails in either concentrated protein solutions at room temperature or in dilute solutions at low temperature. A partially helical monomer was also found and was completely associated with negatively charged detergents in a micelle-bound state. Remarkably, zwitterionic lipids had no effect on the protein structure. By using N- and C-truncated forms of the protein, the C- and N-terminal sequences were identified as possessing an amphiphilic helical character and as selectively associating with acidic detergent micelles. When the study was extended to other chaperonins, it was found that human Cpn10 is also monomeric and partially helical in dilute organic solvent-buffer mixtures. In contrast, Escherichia coli Cpn10 is mostly dimeric and predominately beta-sheet in both dilute and concentrated solutions. Interestingly, human Cpn10 also crosses biological membranes, whereas the E. coli homologue is strictly cytosolic. These results suggest that dissociation to partially helical monomers and interaction with acidic lipids may be two important steps in the mechanism of secretion of M. tuberculosis Cpn10 to the external environment.