Effector loading onto VgrG spike proteins is critical for the assembly of the type VI secretion system in Agrobacterium tumefaciens

The type VI secretion system (T6SS) is used by many bacteria to engage in social behaviors with others and can directly or indirectly affect the health of plants and animals. Because activities associated with T6SS are often costly, the assembly and activation of the T6SS must be highly regulated. However, our knowledge regarding how T6SS assembly and contraction are regulated remains limited. Here we show that the loading of effectors onto their cognate carriers is critical for the assembly of a functional T6SS in Agrobacterium tumefaciens. A. tumefaciens strain C58 encodes one T6SS and two Tde DNase toxin effectors used as major weapons for interbacterial competition. We found that loading of Tde effectors onto their cognate carrier, the VgrG spike, is required for active T6SS secretion. Our data also suggest the assembly of the TssBC contractile sheath occurs only after Tde effectors are loaded onto the VgrG spike. The requirement of effector loading for efficient T6SS secretion was also validated in other A. tumefaciens strains. Such a mechanism may be used by bacteria as a strategy for efficacious T6SS firing. Given the prevalence of T6SS-encoding loci in host-associated bacteria, these findings inform on mechanisms that influence the composition of microbial communities and the services provided to hosts.