Subcellular Localization of the Dot/Icm Type IV Secretion Proteins

Abstract

Legionella pneumophila is the causative agent of Legionnaires’ disease, a fatal form of pneumonia in elderly and immunocompromised people. L. pneumophila replicates inside host cells by using a type IV secretion system (T4SS) encoded by the dot/icm genes. The L. pneumophila dot/icm genes are essential for intracellular multiplication of L. pneumophila. To determine the localization of the L. pneumophila T4SS apparatus, the authors initially investigated the distribution of one Dot protein, DotF. DotF, predicted to localize to the inner membrane by hydrophilicity analysis, has been shown to interact with secreted substrates via a bacterial two-hybrid screen. Quantitative analysis showed that more than 95% of cells had at least one focus of fluorescence at a pole. The majority of the cells showed bipolar DotF staining. The polar staining was specific to DotF because it was not apparent in a strain lacking the dotF gene. Agrobacterium tumefaciensVirB T4SS has been proposed to provide a selective advantage since this plant pathogen attaches to its host cell in a polar manner, thereby allowing intimate contact between the secretion system and the plant cell. Based on the observations, and the work on the A. tumefaciensVirB system, it is possible that polar localization is a conserved feature of all adapted type IV secretion systems. Alternatively, as in the case of adapted conjugation systems and IcsA, there may be a selective advantage or molecular requirement for the proteins to be localized to the bacterial pole.