A conserved adaptor orchestrates co-secretion of synergistic type VI effectors in gut Bacteroidota

Interbacterial competition is crucial for shaping microbial communities and is often mediated by type VI secretion systems (T6SSs) that inject effectors into competing bacteria. T6SS effectors are released via structural proteins such as VgrG, but the secretion timing and coordination are unclear. Here, we report two effectors, BtpeA (Bacteroides T6SS phosphatase effector A) and BtaeB (Bacteroides T6SS amidase effector B), within the Bacteroidota T6SS that exert distinct cell-wall destructive activities critical for interspecies competition but whose secretion is interdependent. BtpeA and BtaeB co-secretion requires an adaptor protein, BtapC (Bacteroides T6SS adaptor protein C), that mediates the sequential assembly of the pre-firing complex, VgrG-BtpeA-BtaeB-BtapC. Structural analyses of this quaternary complex elucidate multi-cargo loading mechanisms with a conserved loop in BtaeB serving as a "checkpoint" to ensure BtpeA co-secretion. During mouse colonization, the combined activities of BtpeA and BtaeB significantly exceed the sum of the individual effectors. These findings unveil a T6SS-mediated co-delivery mechanism that ensures functional synergism of effectors, highlighting potential applications in modulating gut microbiota.