Plasma membrane protrusions mediate host cell-cell fusion induced by Burkholderia thailandensis

Abstract

Cell-cell fusion is important for many biological processes including fertilization, development, immunity, and microbial pathogenesis. The bacterial pathogens in the pseudomallei group of Burkholderia species, including B. thailandensis, are bacteria that spread between host cells by inducing cell-cell fusion. Previous work showed that B. thailandensis-induced cell-cell fusion requires intracellular bacterial motility and a bacterial protein secretion apparatus called the type VI secretion system-5 (T6SS-5), including the T6SS-5 protein VgrG5. However, the cellular level mechanism and T6SS-5 proteins important for cell-cell fusion induced by B. thailandensis remained incompletely described. Using live cell imaging, we found that bacteria used actin-based motility to push on the host cell plasma membrane to form membrane protrusions that extended into neighboring cells. Then, membrane fusion occurred within these membrane protrusions, either proximal to the bacterium at the tip or elsewhere within a protrusion. Expression of VgrG5 by bacteria within membrane protrusions was required to promote cell-cell fusion, as a vgrg5 mutant could not be functionally rescued by VgrG5 secreted by co-infected wild type bacteria. Furthermore, a second predicted T6SS-5 protein, TagD5, was also required for cell-cell fusion. In the absence of VgrG5 or TagD5, bacterial protrusions were engulfed into neighboring cells. Our results suggest that the T6SS-5 effectors VgrG5 and TagD5 are secreted within membrane protrusions and act locally to promote membrane fusion.