Mycobacterium marinum MMAR_2380, a predicted transmembrane acyltransferase, is essential for the presence of the mannose cap on lipoarabinomannan.

Nicole N Driessen,Arun K Mishra,Janneke J Maaskant,Gérald Larrouy-Maumus,Wilbert Bitter,Germain Puzo,Roy Ummels,Jérôme Nigou,Sudagur S Gurcha,Gurdyal S Besra,Esther J M Stoop,Marion Sparrius,Ben J Appelmelk,Martine Gilleron,Christina M J E Vandenbroucke-Grauls

doi:10.1099/mic.0.037507-0

Abstract

Lipoarabinomannan (LAM) is a major glycolipid in the mycobacterial cell envelope. LAM consists of a mannosylphosphatidylinositol (MPI) anchor, a mannan core and a branched arabinan domain. The termini of the arabinan branches can become substituted with one to three α(1→2)-linked mannosyl residues, the mannose cap, producing ManLAM. ManLAM has been associated with a range of different immunomodulatory properties of Mycobacterium tuberculosis during infection of the host. In some of these effects, the presence of the mannose cap on ManLAM appears to be crucial for its activity. So far, in the biosynthesis of the mannose cap on ManLAM, two enzymes have been reported to be involved: a mannosyltransferase that adds the first mannosyl residue of the mannose caps to the arabinan domain of LAM, and another mannosyltransferase that elongates the mannose cap up to three mannosyl residues. Here, we report that a third gene is involved, MMAR_2380, which is the Mycobacterium marinum orthologue of Rv1565c. MMAR_2380 encodes a predicted transmembrane acyltransferase. In M. marinum ΔMMAR_2380, the LAM arabinan domain is still intact, but the mutant LAM lacks the mannose cap. Additional effects of mutation of MMAR_2380 on LAM were observed: a higher degree of branching of both the arabinan domain and the mannan core, and a decreased incorporation of [1,2-14C]acetate into the acyl chains in mutant LAM as compared with the wild-type form. This latter effect was also observed for related lipoglycans, i.e. lipomannan (LM) and phosphatidylinositol mannosides (PIMs). Furthermore, the mutant strain showed increased aggregation in liquid cultures as compared with the wild-type strain. All phenotypic traits of M. marinum ΔMMAR_2380, the deficiency in the mannose cap on LAM and changes at the cell surface, could be reversed by complementing the mutant strain with MMAR_2380. Strikingly, membrane preparations of the mutant strain still showed enzymic activity for the arabinan mannose-capping mannosyltransferase similar to that of the wild-type strain. Although the exact function of MMAR_2380 remains unknown, we show that the protein is essential for the presence of a mannose cap on LAM.

Highlights

The complex mycobacterial cell envelope contains an exceptionally high amount of lipids
In mannose-capped LAM (ManLAM), a part of these terminal arabinosyl residues is substituted with an a(1A5)-linked mannosyl residue which can be further elongated by the addition of one or two a(1A2)-linked mannosyl residues, forming the mannose cap (Supplementary Fig. S1)
We reveal a third gene involved in the biosynthesis of the mannose cap on LAM

Summary

Introduction

The complex mycobacterial cell envelope contains an exceptionally high amount of lipids. Additional effects of the mutation of MMAR_2380 were seen in a higher degree of branching of both the arabinan domain and the mannan core in mutant LAM, decreased [1,2-14C]acetate incorporation in the acyl chains of mutant LAM and related lipoglycans, and an altered cell surface of the mutant strain as compared with wild-type M. marinum.

Results

Conclusion