The archetype Pseudomonas aeruginosa proteins TssB and TagJ form a novel subcomplex in the bacterial type VI secretion system.

In Pseudomonas aeruginosa three type VI secretion systems (T6SSs) coexist, called H1- to H3-T6SSs. Several T6SS components are proposed to be part of a macromolecular complex resembling the bacteriophage tail. The T6SS protein HsiE1 (TagJ) is unique to the H1-T6SS and absent from the H2- and H3-T6SSs. We demonstrate that HsiE1 interacts with a predicted N-terminal α-helix in HsiB1 (TssB) thus forming a novel subcomplex of the T6SS. HsiB1 is homologous to the Vibrio cholerae VipA component, which contributes to the formation of a bacteriophage tail sheath-like structure. We show that the interaction between HsiE1 and HsiB1 is specific and does not occur between HsiE1 and HsiB2. Proteins of the TssB family encoded in T6SS clusters lacking a gene encoding a TagJ-like component are often devoid of the predicted N-terminal helical region, which suggests co-evolution. We observe that a synthetic peptide corresponding to the N-terminal 20 amino acids of HsiB1 interacts with purified HsiE1 protein. This interaction is a common feature to other bacterial T6SSs that display a TagJ homologue as shown here with Serratia marcescens. We further show that hsiE1 is a non-essential gene for the T6SS and suggest that HsiE1 may modulate incorporation of HsiB1 into the T6SS.