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Open Access Highly Accessed Research article

Lipoproteins of slow-growing Mycobacteria carry three fatty acids and are N-acylated by Apolipoprotein N-Acyltransferase BCG_2070c

Juliane K Brülle1, Andreas Tschumi1 and Peter Sander12*

Author Affiliations

1 Institute of Medical Microbiology, University of Zurich, Gloriastrasse 30/32, CH-8006, Zurich, Switzerland

2 National Reference Laboratory for Mycobacteria (NRLM), Gloriastrasse 30, CH-8006, Zurich, Switzerland

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BMC Microbiology 2013, 13:223  doi:10.1186/1471-2180-13-223

Published: 5 October 2013

Abstract

Background

Lipoproteins are virulence factors of Mycobacterium tuberculosis. Bacterial lipoproteins are modified by the consecutive action of preprolipoprotein diacylglyceryl transferase (Lgt), prolipoprotein signal peptidase (LspA) and apolipoprotein N- acyltransferase (Lnt) leading to the formation of mature triacylated lipoproteins. Lnt homologues are found in Gram-negative and high GC-rich Gram-positive, but not in low GC-rich Gram-positive bacteria, although N-acylation is observed. In fast-growing Mycobacterium smegmatis, the molecular structure of the lipid modification of lipoproteins was resolved recently as a diacylglyceryl residue carrying ester-bound palmitic acid and ester-bound tuberculostearic acid and an additional amide-bound palmitic acid.

Results

We exploit the vaccine strain Mycobacterium bovis BCG as model organism to investigate lipoprotein modifications in slow-growing mycobacteria. Using Escherichia coli Lnt as a query in BLASTp search, we identified BCG_2070c and BCG_2279c as putative lnt genes in M. bovis BCG. Lipoproteins LprF, LpqH, LpqL and LppX were expressed in M. bovis BCG and BCG_2070c lnt knock-out mutant and lipid modifications were analyzed at molecular level by matrix-assisted laser desorption ionization time-of-flight/time-of-flight analysis. Lipoprotein N-acylation was observed in wildtype but not in BCG_2070c mutants. Lipoprotein N- acylation with palmitoyl and tuberculostearyl residues was observed.

Conclusions

Lipoproteins are triacylated in slow-growing mycobacteria. BCG_2070c encodes a functional Lnt in M. bovis BCG. We identified mycobacteria-specific tuberculostearic acid as further substrate for N-acylation in slow-growing mycobacteria.

Keywords:
Mycobacteria; Tuberculosis; Lipoprotein; Lipidation; Apolipoprotein N-acyltransferase