Research article. Dual control of host actin polymerization by a Legionella effector pair

Host actin cytoskeleton is often targeted by pathogenic bacteria through the secretion of effectors. Legionella pneumophila virulence relies on the injection of the largest known arsenal of bacterial proteins, over 330 Dot/Icm Type 4 Secretion System effectors, into the host cytosol. Defining the functional role of bacterial toxins that has antagonistic activities towards actin polymerization has been challenging. Here we define the functional interactions between VipA and LegK2, two effectors targeting actin polymerization that have been proposed to interfere with the endosomal pathway. Bacteria lacking Legk2 effector had defects (i) in the inhibition of actin polymerization at the Legionella Containing Vacuole, (ii) in endosomal escape of the bacteria and (iii) intracellular replication. Additionally, we observed the restoration of the legK2 mutant defects, upon deletion of vipA, making LegK2/VipA the first example of effector-effector suppression pair that targets the actin cytoskeleton and whose functional interaction impacts L. pneumophila virulence. We demonstrated that LegK2 and VipA do not modulate each other's activity in a classical ‘metaeffector’ relationship. Instead, the antagonistic activities of the LegK2/VipA effector pair would target different substrates, Arp2/3 for LegK2 and G-actin for VipA, to temporally control actin polymerization at the LCV and interfere with phagosome maturation and endosome recycling, thus contributing to the intracellular life cycle of the bacterium. Strikingly, the functional interaction between LegK2 and VipA is consolidated by an evolutionary history that has refined the effector repertoire of L. pneumophila. KEYWORDS: Legionella pneumophila; secretion effectors; effector-effector functional interaction; host actin cytoskeleton; endosomal pathway; LegK2; VipA; effector repertoire