Identification of a novel α(1→6) mannopyranosyltransferase MptB from Corynebacterium glutamicum by deletion of a conserved gene, NCgl1505, affords a lipomannan- and lipoarabinomannan-deficient mutant

Mycobacterium tuberculosis and Corynebacterium glutamicum share a similar cell wall structure and orthologous enzymes involved in cell wall assembly. Herein, we have studied C. glutamicum NCgl1505, the orthologue of putative glycosyltransferases Rv1459c from M. tuberculosis and MSMEG3120 from Mycobacterium smegmatis. Deletion of NCgl1505 resulted in the absence of lipomannan (Cg-LM-A), lipoarabinomannan (Cg-LAM) and a multi-mannosylated polymer (Cg-LM-B) based on a 1,2-di-O-C-16/C-18:1-(alpha-D-glucopyranosyluronic acid)-(1 -> 3)-glycerol (GlcAGroAc(2)) anchor, while syntheses of triacylated-phosphatidyl-myo-inositol dimannoside (Ac1PIM2) and Man(1)GlcAGroAc(2) were still abundant in whole cells. Cell-free incubation of C. glutamicum membranes with GDP-[C-14]Man established that C. glutamicum synthesized a novel alpha(1 -> 6)-linked linear form of Cg-LM-A and Cg-LM-B from Ac1PIM2 and Man(1)GlcAGroAc(2) respectively. Furthermore, deletion of NCgl1505 also led to the absence of in vitro synthesized linear Cg-LM-A and Cg-LM-B, demonstrating that NCgl1505 was involved in core alpha(1 -> 6) mannan biosynthesis of Cg-LM-A and Cg-LM-B, extending Ac1PI[C-14]M-2 and [C-14]Man(1)GlcAGroAc(2) primers respectively. Use of the acceptor alpha-D-Manp-(1 -> 6)-alpha-D-Manp-O-C-8 in an in vitro cell-free assay confirmed NCgl1505 as an alpha(1 -> 6) mannopyranosyltransferase, now termed MptB. While Rv1459c and MSMEG3120 demonstrated similar in vitro alpha(1 -> 6) mannopyranosyltransferase activity, deletion of the Rv1459c homologue in M. smegmatis did not result in loss of mycobacterial LM/LAM, indicating a functional redundancy for this enzyme in mycobacteria.