L. pneumophila proliferates within alveolar macrophages as a central property of Legionnaires’ disease. Intracellular growth involves formation of a replication vacuole, which requires the bacterial Dot/Icm system, a multi-protein secretion apparatus that translocates proteins from the bacterium across the macrophage plasma membrane. Two components of this system, IcmR and IcmQ, have been previously shown to be binding partners and fractionate to the cytoplasm of the bacterium. We report here that IcmQ both inserts into and disrupts in vitro prepared phospholipid membranes allowing the efflux of the dye calcein. Both membrane insertion and disruption are inhibited in the presence of IcmR. Trypsin digestion mapping demonstrated that IcmQ is subdivided into two functional domains. The amino-terminal region of IcmQ was necessary and sufficient for insertion into lipid membranes and calcein efflux. The carboxy terminal domain was necessary for efficient association of the protein with lipid bilayers. IcmR was found to bind to the amino terminal portion of the protein thus providing a mechanism for its ability to inhibit IcmQ interaction with membranes.

In addition, we report that IcmQ interacts with three additional proteins SidA, Lpg0258, and Lpg1969 with high affinity. These proteins are secreted in a Dot/Icm dependent manner although they appear to not be required for intracellular growth. Binding of IcmQ by these proteins appears to require only the C-terminal region of the protein and is inhibited in the presence of IcmR. This data along with membrane fractionation data suggest a role for IcmQ in the membrane portion of Dot/Icm Type IV Secretion System. Upon exposure of L. pneumophila to host cells, IcmQ may release from IcmR and move to the Dot/Icm complex where it inserts into the bacterial inner membrane and interacts DotF. Here, IcmQ may act to recruit Dot/Icm dependent secreted proteins, like SidA, for secretion by the Dot/Icm complex.