Functional and mechanistic characterization of the phosphatidyl-myo-inositol mannosyltransferase PimA from...

Human-adapted Mycobacterium tuberculosis (Mtb) also infects animals and transmits zoonotically, threatening public health. The cell wall glycolipids lipoarabinomannan and lipomannan are essential virulence factors of Mtb. Phosphatidyl-myo-inositol mannosyltransferase PimA catalyzes a key step in their biosynthesis and represents a potential drug target. However, the functional mechanism of Mtb PimA remains incompletely understood. Here, the hydrodynamic diameter of monomeric PimA was measured as 5.1 ± 0.2 nm. The structural model of PimA was predicted using AlphaFold2. Molecular docking was conducted to predict potential binding sites, and site-directed mutagenesis was then performed. Mutations D253A, E274A or E282A dramatically reduced the activity, whereas R196A, K202A or K256A abolished activity entirely. Guanosine diphosphate (GDP) and benzimidazole inhibited PimA by over 50% at 40 µM and 10 µM, respectively. PimA transcription increased linearly over time in macrophages, and conditional PimA-silenced mutant rendered Mtb rapidly susceptible to macrophage killing. These results provide a structural and functional framework for the rational design of novel anti-tubercular agents targeting PimA.