A T4SS effector directly disarms competing antibacterial T6SSs by hijacking their conserved spike component

Interspecies competition plays a crucial role in shaping microbial community dynamics and influencing host-associated outcomes. However, the mechanisms by which microbes directly neutralize the antimicrobial systems of their rivals remain largely unexplored. Here, we demonstrate that a bacterial effector protein, translocated via the widely distributed type IV secretion system (T4SS), directly disarms the antibacterial type VI secretion system (T6SS) of a competitor. Specifically, we show that Lysobacter enzymogenes, a ubiquitous soil bacterium, uses its bacterial-killing T4SS (T4SS_BK) to deliver a non-lethal effector, LqqE1, into the cytoplasm of Pseudomonas putida, a competitor equipped with a functional antibacterial K1-T6SS. LqqE1 targets and hijacks VgrG1, a conserved structural component of the K1-T6SS's spike in P. putida, through direct binding. We propose that the binding of LqqE1 to VgrG1 disrupts the native loading of the K1-T6SS nuclease effector Tke2 onto VgrG1. As a result, the translocated LqqE1 abolishes the secretion of the inner-tube protein Hcp via K1-T6SS, effectively interfering with the antibacterial function of P. putida. Structural and phylogenetic analyses reveal that LqqE1 homologs are widespread among bacteria that encode T4SS, with several representative members exhibiting similar K1-T6SS-disabling functions. These findings uncover a novel interbacterial warfare strategy in which a T4SS_BK effector sabotages a competitor's T6SS by subverting its core structural architecture. Our results provide new insights into the molecular arms race between two distinct and widespread antibacterial contact-dependent secretion systems, enhancing our understanding of the diversity in microbial community competition.