Biogenic Insecticides decoded: Scientists from Bochum and Freiburg present their results in Science
Infective entomopathogenic nematodes are minute worms that penetrate insect larva and serve as transport vehicles for the bacterium Photorhabdus luminescens, which lives in their alimentary tract.
After larva penetration the nematodes release these bacteria, which secrete toxins that compromise the immune system of the larvae. Thus the nematodes are used together with the bacterium as insecticide. The exact mechanism of action of the Photorhabdus luminescens had been unknown to date.
Together with colleagues from Freiburg, Prof. em. Hans Georg Mannherz (Medical Faculty of the RUB and the Max-Planck-Institute for Molecular Physiology, Dortmund) has now managed to elucidate this process. Specific subunits of the bacterial toxin complex that inhibit essential defence reactions of immune cells are of instrumental significance. Some of the bacterial infectious toxins bear resemblances to toxins of human pathogenic bacteria, e.g. the bacteria that cause pulmonary and bubonic plaque may make use of similar mechanisms. The researchers have published their findings in the current volume of SCIENCE.
Worms transport the bacteria to the target
Photorhabdus luminescens lives in symbiosis with nematodes. The minute worms penetrate insect larvae via natural openings and then more or less “regurgitate” the bacteria. Bacterial toxins produced by this light-emitting bacterium kill the insect larvae thus creating a large food reservoir for the proliferation of nematodes and bacteria.
Two subunits of the toxin complex are biologically active
Photorhabdus luminescens produces diverse toxins that generate large toxin complexes (Tc proteins). The biologically active complex consists of the three components TcA, TcB and TcC. So far the mechanism of action of these toxins has been unknown. Together with the Dow AgroSciences (USA) and Prof. em. Mannherz and research scientists in Freiburg, working under the auspices of Prof. Klaus Aktories and Prof. Gudula Schmidt, investigated the impact of the toxins on insect and mammalian cells. They were able to demonstrate that the biological activity is located in the TcC components TccC3 and TccC5. The two toxin components are enzymes that inhibit the essential defence reactions of immune cells, e.g. the phagocytosis of bacteria.
Toxins function in two ways
The toxins act on the target cells of the insect larvae in two different ways. TccC3 modifies the cytoskeletal protein actin (ADP-ribosylation) to such an extent that it is no longer controlled by the regulator protein thymosin beta 4. This results in a significant polymerisation of the actin. The second toxin, TccC5, changes so-called Rho proteins, i.e. the switch proteins responsible for the regulation of the actin cytoskeleton. Normally these regulators are switched on and off within the cell. TccC5 modifies the switch by blocking the switching-off procedure. Subsequently the permanently active Rho protein enhances the polymerisation of actin. Together the two toxins lead to a strong aggregation and even clustering of the actin cytoskeleton, which is incompatible with the normal cellular function or immune defence reaction. TcA, which forms pores in host cells, is necessary to enable the toxins TccC3 and TccC5 to enter the insect cells. The toxins probably infiltrate the interior of the cells through these pores.
Decisive knowledge for the comprehension of Tc proteins
Tc Proteins have also been identified in human pathogenic bacteria such as Yersinia pseudotuberculosis and Yersinia pestis. As Prof. Mannherz pointed out, the clarification of the molecular mechanism of prototypical Tc proteins is thus of fundamental importance for the comprehension of other Tc proteins from insecticidal and human pathogenic bacteria.
Alexander E. Lang, Gudula Schmidt, Andreas Schlosser, Timothy D. Hey, Ignacio M. Larrinua, Joel J. Sheets, Hans G. Mannherz and Klaus Aktories: “Photorhabdus luminescens Toxins ADP-Ribosylate Actin and RhoA to Force Actin Clustering.” In: Science 26 February 2010 327: 1139-1142 [DOI: 10.1126/science.1184557]
Prof. em. Hans Georg Mannherz, Abteilung für Anatomie und Embryologie der Ruhr-Universität Bochum und Abteilung für Physikalische Biochemie des Max-Planck-Instituts für Molekulare Physiologie, Dortmund, Germany
Editor: Meike Drießen
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