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-C₁₆/C_18:1-(α- D-glucopyranosyluronic acid)-(1→3)-glycerol (GlcAGroAc₂) anchor, while syntheses of triacylated-phosphatidyl-myo-inositol dimannoside (Ac ₁PIM₂) and Man₁GlcAGroAc₂ were still abundant in whole cells. Cell-free incubation of C. glutamicum membranes with GDP-[¹⁴C]Man established that C. glutamicum synthesized a novel α(1→6)-linked linear form of Cg-LM-A and Cg-LM-B from Ac ₁PIM₂ and Man₁GlcAGroAc₂ 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 α(1→6) mannan biosynthesis of Cg-LM-A and Cg-LM-B, extending Ac₁PI[¹⁴C]M₂ and [ ¹⁴C]Man₁GlcAGroAc₂ primers respectively. Use of the acceptor α-D-Manp-(1→6)-α-D-Manp-O-C₈ in an in vitro cell-free assay confirmed NCgl1505 as an α(1→6) mannopyranosyltransferase, now termed MptB. While Rv1459c and MSMEG3120 demonstrated similar in vitro α(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.