PknG kinase promotes mycobacterial infection by phosphorylating host KARS to regulate the MAPK signaling axis.

Tuberculosis remains one of the deadliest infectious diseases worldwide, and the spread of drug-resistant strains has created an urgent need for novel therapeutic targets. In a study published in iMetaMed, researchers from the Zhongshan Institute for Drug Discovery, Shanghai Institute of Materia Medica, Chinese Academy of Sciences – led by the teams of Dr. Junyu Xu and Dr. Minjia Tan – have uncovered a previously unknown mechanism by which mycobacteria modulate host immune responses.

The team found that the bacterial serine/threonine protein kinase PknG directly targets the host protein lysine‑tRNA ligase (KARS) and phosphorylates it at two specific sites, T592 and T595. This phosphorylation event subsequently modulates the host MAPK/NF‑κB signaling pathway, dampens immune responses, and helps the pathogen survive inside macrophages.

"Our study is the first to identify KARS as a direct substrate of mycobacterial PknG," explained from Dr. Xu’s team. "By pinpointing the T592 and T595 phosphorylation sites, we provide a potential new drug target for host‑directed therapy against mycobacteria."

Using quantitative proteomics and phosphoproteomics, the team compared cells overexpressing PknG with control cells. The results showed that PknG‑mediated KARS phosphorylation not only altered MAPK/NF‑κB signaling but also induced profound changes in immune cell populations. In an animal model, PknG overexpression led to elevated mRNA levels of pro‑inflammatory cytokines Tnfα, Il1b, and Il6, accompanied by more severe histopathological lung damage.

Flow cytometry further revealed that M. smegmatis expressing Mtb PknG significantly reduced splenic CD3⁺ and CD8⁺ T cells, indicating an immunosuppressive trend. In contrast, a strain expressing PknG from Mycobacterium smegmatis increased the proportion of myeloid‑derived suppressor cells (MDSCs) and multiple regulatory T‑cell subsets, altered CD8⁺ T cell composition, and exhibited enhanced regulatory immunity and remodelling of T cell subsets.

"These results indicate that the PknG‑KARS axis is a central hub through which mycobacteria remodel the host immune network," said the lead investigator from Dr. Xu’s team. "Targeting the T592 and T595 sites of KARS could disrupt this bacterial immune evasion strategy without directly killing the bacteria – a potential approach to overcome drug resistance."

The findings not only advance our understanding of tuberculosis pathogenesis but also reveal that host proteins such as KARS can be hijacked by pathogens for immune regulation. The researchers are now exploring small‑molecule inhibitors that block PknG‑mediated KARS phosphorylation, aiming to develop a new class of therapeutics.