The Type VI Secretion System Spike Protein VgrG5 Mediates Membrane Fusion during Intercellular Spread by Pseudomallei Group Burkholderia Species

Abstract

ABSTRACTPseudomallei groupBurkholderiaspecies are facultative intracellular parasites that spread efficiently from cell to cell by a mechanism involving the fusion of adjacent cell membranes. Intercellular fusion requires the function of the cluster 5 type VI secretion system (T6SS-5) and its associated valine-glycine repeat protein, VgrG5. Here we show that VgrG5 alleles are conserved and functionally interchangeable betweenBurkholderia pseudomalleiand its relativesB. mallei,B. oklahomensis, andB. thailandensis. We also demonstrate that the integrity of the VgrG5 C-terminal domain is required for fusogenic activity, and we identify sequence motifs, including two hydrophobic segments, that are important for fusion. Mutagenesis and secretion experiments usingB. pseudomalleistrains engineered to express T6SS-5in vitroshow that the VgrG5 C-terminal domain is dispensable for T6SS-mediated secretion of Hcp5, demonstrating that the ability of VgrG5 to mediate membrane fusion can be uncoupled from its essential role in type VI secretion. We propose a model in which a unique fusogenic activity at the C terminus of VgrG5 facilitates intercellular spread byB. pseudomalleiand related species following injection across the plasma membranes of infected cells.