Forschung

RpkA (Receptor Phosphatidylinositol Kinase A)
an unusual seven-helix transmembrane protein of Dictyostelium discoideum with a GPCR signature and a C-terminal lipid kinase domain (GPCR-PIPK) is highly conserved in lower eukaryotes like in the well-known Legionella host Acanthamoeba but is absent in higher eukaryotes (Bakthavatsalam et al., 2007; Bakthavatsalam et al., 2006). RpkA localizes to endosomes, especially to late endosomes and lysosomes. RpkA is specifically transported to maturing phagosomes with similar kinetics as V-ATPase and interacts with this protein complex. The overall pH however remains unaffected in the rpkA .
Loss of RpkA leads to a reduced turnover of PIP and PIP2 as well as to a reduced nitrogen starvation tolerance which is a hallmark for autophagy.

RpkA is involved in the defense against Legionella
The gram-negative bacterium L. pneumophila is the pathogenic agent of Legionnaires' disease. Upon inhalation of contaminated aerosols it hijacks pulmonary macrophages in human hosts by reprogramming their phagosomes to become Legionella-containing vacuoles (LCVs). Still, humans are only one possible host and a "dead end street" for the bacteria because horizontal transmission from human to human does not occur (Fraser et al., 1977). In fact, primary targets of L. pneumophila are free living amoebae (FLA) in which the bacterium lives, divides and foremost is able to switch to a highly infectious mature intracellular form (MIF) which only occurs when grown in amoeba (Garduno et al., 2002). Within FLA Legionella can sustain even antibacterial environments, enabling Legionella to colonize artificial water systems by using FLA as vectors. Hereby the bacterium is positioned to infect humans. Therefore, it is worth investigating not only the host-pathogen interactions of Legionella in human macrophages but also in amoebae. This will provide an opportunity to intervene in this relationship and to reduce the outbreak of Legionellosis. Several host targets affected by Legionella effectors may be similar in amoebae and macrophages; however there are also differences especially regarding defense mechanisms.
The KO of RpkA leads to less effective killing of wild type L. pneumophila and even L. hackeliae can survive significantly longer in rpkA than in the wild type making RpkA a component of the defense system of D. discoideum.


Figure 1: RpkA co-localizes with the V-ATPase at the Legionella containing phagosome L. pneumophila is stained with DAPI (arrow head). The bacterium is in a phagosome which contains RpkA and V-ATPase (VatA).

Literature:
Bakthavatsalam, D., Brazill, D., Gomer, R. H., Eichinger, L., Rivero, F., and Noegel, A. A. (2007). A G protein-coupled receptor with a lipid kinase domain is involved in cell-density sensing. Curr Biol 17, 892-897.

Bakthavatsalam, D., Meijer, H. J., Noegel, A. A., and Govers, F. (2006). Novel phosphatidylinositol phosphate kinases with a G-protein coupled receptor signature are shared by Dictyostelium and Phytophthora. Trends in microbiology 14, 378-382.

Fraser, D. W., Tsai, T. R., Orenstein, W., Parkin, W. E., Beecham, H. J., Sharrar, R. G., Harris, J., Mallison, G. F., Martin, S. M., McDade, J. E., et al. (1977). Legionnaires' disease: description of an epidemic of pneumonia. N Engl J Med 297, 1189-1197.

Garduno, R. A., Garduno, E., Hiltz, M., and Hoffman, P. S. (2002). Intracellular growth of Legionella pneumophila gives rise to a differentiated form dissimilar to stationary-phase forms. Infection and immunity 70, 6273-6283.