Deciphering Legionella effector delivery by Icm/Dot secretion system reveals a new role for c-diGMP signaling

Abstract

Secretion of bacterial effector proteins into host cells plays a key role in bacterial virulence. Yet, the dynamics of the secretion systems activity remains poorly understood, especially when machineries deal with the export of numerous effectors. We address the question of multi-effector secretion by focusing on the Legionella pneumophila Icm/Dot T4SS that translocates a record number of 300 effectors. We set up a kinetic translocation assay, based on the {beta}-lactamase translocation reporter system combined with the effect of the protonophore CCCP. When used for translocation analysis of Icm/Dot substrates constitutively produced by L. pneumophila, this assay allows a fine monitoring of the secretion activity of the T4SS, independently of the expression control of the effectors. We observed that effectors are translocated with a specific timing, suggesting a control of their docking/translocation by the T4SS. Their delivery is accurately organized to allow effective manipulation of the host cell, as exemplified by the sequential translocation of effectors targeting Rab1, namely SidM/DrrA, LidA, LepB. Remarkably, the timed delivery of effectors does not depend only on their interaction with chaperone proteins but implies cyclic-di-GMP signaling, as the diguanylate cyclase Lpl0780/Lpp0809, contributes to the timing of translocation. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=179 SRC="FIGDIR/small/754762v2_ufig1.gif" ALT="Figure 1"> View larger version (38K): org.highwire.dtl.DTLVardef@17b7245org.highwire.dtl.DTLVardef@18b2443org.highwire.dtl.DTLVardef@1f9b7ceorg.highwire.dtl.DTLVardef@19275ff_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIThe dynamics of multi-effector secretion is addressed in the paradigm Legionella Icm/Dot T4SS C_LIO_LIA kinetic translocation assay allowing a fine monitoring of the T4SS activity is set up C_LIO_LISpecific secretion patterns consistent with sequential functions in the host are reported C_LIO_LIT4SS-dependent translocation is promoted by a diguanylate cyclase C_LIO_LIUnreported control of Type 4 secretion by the second messenger c-di-GMP signaling is revealed C_LI