The type VI secretion system (T6SS) has emerged as a critical virulence factor for the group of closely related Burkholderia spp. that includes B. pseudomallei, B. mallei and B. thailandensis. While the genomes of these bacteria, referred to as Bptm, appear to encode several T6SSs, we and others have shown that one of these, type VI secretion system 5 (T6SS-5), is required for their virulence in mammalian infection models. Despite its pivotal role in the pathogenesis of Bptm, the effector repertoire of T6SS-5 has remained elusive. Here we used quantitative mass spectrometry to compare the secretome of wildtype B. thailandensis to a mutant harboring a non-functional T6SS-5. This analysis identified VgrG-5 as a n... More
The type VI secretion system (T6SS) has emerged as a critical virulence factor for the group of closely related Burkholderia spp. that includes B. pseudomallei, B. mallei and B. thailandensis. While the genomes of these bacteria, referred to as Bptm, appear to encode several T6SSs, we and others have shown that one of these, type VI secretion system 5 (T6SS-5), is required for their virulence in mammalian infection models. Despite its pivotal role in the pathogenesis of Bptm, the effector repertoire of T6SS-5 has remained elusive. Here we used quantitative mass spectrometry to compare the secretome of wildtype B. thailandensis to a mutant harboring a non-functional T6SS-5. This analysis identified VgrG-5 as a novel secreted protein whose export depends upon T6SS-5 function. Bioinformatics analysis revealed VgrG-5 is a specialized VgrG protein that harbors a C-terminal domain (CTD) conserved among Bptm. We found that a vgrG-5 ΔCTD mutant is avirulent in mice and unable to stimulate fusion of host cells, a hallmark of Bptm previously shown to require T6SS-5 function. The singularity of VgrG-5 as a detected T6SS-5 substrate, taken together with the essentiality of its CTD for virulence, suggests that the protein is a critical for effector activity of T6SS-5. Intriguingly, we show that unlike the bacterial cell-targeting T6SSs so far characterized, T6SS-5 localizes to the bacterial cell pole. We propose a model whereby the CTD of VgrG-5 - propelled by T6SS-5 - plays a key role in inducing membrane fusion, either by the recruitment of other factors or by direct participation.
