The Legionella pneumophila IcmR protein exhibits chaperone activity for IcmQ by preventing its participation in high-molecular-weight complexes

A key event in legionellosis is the ability of Legionella pneumophila to su rvive and proliferate inside alveolar macrophages. The dot/icm genes, which are necessary for intracellular growth, show sequence similarity to genes encoding conjugative transfer systems, and it is believed that they are res ponsible for the formation of a secretion apparatus. Evidence is provided h ere that the IcmR and IcmQ proteins participate in a chaperone-substrate re lationship similar to that observed for translocated proteins in type III a nd type IV secretion apparatuses. Immobilized IcmQ was found to bind IcmR f rom crude bacterial extracts efficiently. Furthermore, purified IcmR and Ic mQ bind with high affinity. This interaction was also observed in vivo by c o-immunoprecipitation. The presence of IcmR was found to affect the physica l state of IcmQ directly. In the absence of IcmR, IcmQ formed high-molecula r-weight complexes both in vivo and in vitro, whereas IcmR prevented and re versed the formation of these complexes.