1.B.167.  The Tube-forming Protein in the Mycobacterial Envelop (TiME) Family 

Tuberculosis-causing mycobacteria have thick cell-wall and capsule layers. Protein secretion across these barriers depends on a specialized protein secretion system. Cai et al. 2021 showed that Mycobacterium tuberculosis Rv3705c and its homologous MSMEG_6251 in Mycobacterium smegmatis are tube-forming proteins in the mycobacterial envelope (TiME). Crystallographic and cryo-EM structures of these two proteins showed that both proteins form rotationally symmetric rings. Two layers of TiME rings pack together in a tail-to-tail manner into a ring-shaped complex, which, in turn, stacks together to form tubes. M. smegmatis TiME was detected mainly in the cell wall and capsule. Knocking out the TiME gene markedly decreased the amount of secreted protein in the M. smegmatis culture medium, and expression of this gene in a knocked-out strain partially restored the level of secreted protein. These structure and functional data thus suggest that TiME forms a protein transport tube across the mycobacterial outer envelope (Cai et al. 2021).

Liu et al. 2024 focused on the model organism Mycolicibacterium smegmatis, which has a cell envelope similar to that of Mycobacterium tuberculosis, as well as the TiME protein secretion tube across the mycobacterial outer envelope. Transcriptome results and analyzes of the protein compositions of a mycobacterial surface envelope, determining that more transporters and porins are induced to complement the deletion of the time gene in M. smegmatis. The TiME protein is essential for nutrient utilization, as demonstrated in the uptake experiments and growth on various monosaccharides or with amino acids as the sole carbon source. Its deletion caused bacteria to be more sensitive to anti-TB drugs and to show a growth defect at an acid pH level, indicating that TiME promotes the survival of M. smegmatis in antibiotic-containing and acidic environments. These results suggest that TiME tubes facilitate bi-directional processes for both protein secretion and nutrient uptake across the mycobacterial outer envelope.(Liu et al. 2024).