Tvp complexes support formation of the VgrG-PAAR spike during Type VI secretion system assembly

Abstract

Type VI secretion systems (T6SSs) are nanomachineries used by bacteria to inject toxic effectors into neighbouring cells during interbacterial competition or infection. The existence of multiple paralogues of the structural proteins VgrG and PAAR and of specific accessory proteins allows the same T6SS to deliver a wide range of effectors. We describe a new set of accessory proteins required for assembly of T6SSs containing the VgrG1-Paar1 spike and delivery of a novel VgrG1-associated membrane-targeting effector in Serratia marcescens. TvpAB, TvpB, TvpC and Paar1 form a pre-complex essential for T6SS assembly, with VgrG1 subsequently replacing TvpC. Phylogenetic analysis and structural modelling reveal that Tvp proteins are widely conserved but Tvp-containing complexes vary in organisation and complexity. We define three classes of Tvp complex, all sharing a DUF2169-containing TvpA protein which stabilises a DUF4150/PAAR-containing protein. Class I, which includes only TvpA, functions without a pre-complex, whereas Classes II and III involve additional Tvp components and two-step assembly. Our findings highlight how modular effector recruitment strategies underlie the versatility of the T6SS and suggest how alternative Tvp systems are tailored to promote assembly of secretion-competent, effector-loaded VgrG-PAAR spikes.