Shigella hijacks the exocyst to cluster macropinosomes for efficient vacuolar escape

Shigella flexneri invades host cells by entering within a bacteria-containing vacuole (BCV). In order to establish its niche in the host cytosol, the bacterium ruptures its BCV. Contacts between S. flexneri BCV and infection-associated macropinosomes (IAMs) formed in situ have been reported to enhance BCV disintegration. The mechanism underlying S. flexneri vacuolar escape remains however obscure. To decipher the molecular mechanism priming the communication between the IAMs and S. flexneri BCV, we performed mass spectrometry-based analysis of the magnetically purified IAMs from S. flexneri-infected cells. While proteins involved in host recycling and exocytic pathways were significantly enriched at the IAMs, we demonstrate more precisely that the S. flexneri type III effector protein IpgD mediates the recruitment of the exocyst to the IAMs through the Rab8/Rab11 pathway. This recruitment results in IAM clustering around S. flexneri BCV. More importantly, we reveal that IAM clustering subsequently facilitates an IAM-mediated unwrapping of the ruptured vacuole membranes from S. flexneri, enabling the naked bacterium to be ready for intercellular spread via actin-based motility. Taken together, our work untangles the molecular cascade of S. flexneri-driven host trafficking subversion at IAMs to develop its cytosolic lifestyle, a crucial step en route for infection progression at cellular and tissue level.
Author summary Shigella flexneri is a clinically relevant bacterial pathogen that causes bacillary dysentery. It invades the host cell by injecting a repository of bacterial effectors through its type III secretion system. Upon its entry into host cell, S. flexneri resides shortly in a bacteria-containing vacuole (BCV), which is rapidly ruptured for the cytosolic propagation and infection progression of the pathogen. Infection-associated macropinosomes (IAMs) formed in situ during S. flexneri entry are found to promote the efficient vacuolar escape of S. flexneri with an unclear mechanism. We present here the molecular players involving in the BCV-IAM interactions obtained by proteomic analysis of magnetically-purified IAMs. We decipher the successive steps of S. flexneri BCV escape, pinpointing the bacterial effector-mediated hijacking of host trafficking pathways to promote the BCV disintegration and displacement of BCV membranes from the bacteria. This study sheds light on the mechanism by which bacterial pathogens modulate BCV vacuolar integrity through interaction with infection-induced vesicle compartments for the establishment of their intracellular replicative niches and pathogenicities.