A putative mycobacterial GDP-mannose dependent α-mannosyltransferase Rv0225 acts as PimC: an in-silico study

The complex cell envelope of pathogenic mycobacteria provides a strong barrier against the host immune system and various antibiotics. Phosphatidyl-myo-inositol mannosides (PIMs), lipomannan (LM), and lipoarabinomannan (LAM) are structurally important elements of mycobacterial cell envelope and also play crucial roles in modulating the host immune functions. At the cytoplasmic side of the mycobacterial inner membrane, phosphatidyl-myo-inositol (PI) is mannosylated by α-mannosyltransferases PimA and PimB' to synthesize PIM₂ using GDP-mannose (GDPM) as the mannose donor. This PIM₂ compound is acylated to synthesize Ac_1/2PIM₂, which is further mannosylated by an unknown enzyme PimC to produce Ac_1/2PIM₃. Synthesis of LM/LAM or higher PIM compounds (Ac_1/2PIM₄ / Ac_1/2PIM₅ / Ac_1/2PIM₆) requires polyprenol-phosphate-mannose (PPM) as the mannose donor and takes place at the periplasmic side of the mycobacterial inner membrane. Previously, a GDPM-dependent α-mannosyltransferase RvD2-ORF1 was identified as the PimC in Mycobacterium tuberculosis CDC1551 (Mtb CDC1551). However, its counterpart was missing in most other mycobacterial strains. Bioinformatic analyses, molecular docking, and molecular dynamics (MD) simulations in this study indicate that Rv0225, an essential protein of Mycobacterium tuberculosis H37Rv, is a GDPM-binding α-mannosyltransferase. The predicted structure of Rv0225 showed similarities with mycobacterial proteins PimA, PimB', and PimC of Mtb CDC1551. Further molecular docking and MD simulations also suggest that Ac_1/2PIM₂ can bind to Rv0225 and showed similar dynamic patterns as the glycolipid substrates of PimA and PimB'. The binding of Ac₁PIM₃ caused opening and closing motions of Rv0225, a phenomenon also observed in the case of PimA. Overall, the computational analyses suggest that Rv0225 may play the role of PimC in mycobacteria.