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Bacterial gene of balance

Targeted attack on storage proteins regulates iron homoeostasis in bacteria

Jülich, 21st June 2016 – Some bacteria contain a gene which encode for a protein that label other proteins by “pupylation” for the degradation of these target proteins. Studies from our institute IBG-1: Biotechnology showed, that pupylation of a target protein also serves for another function than only the labelling of proteins for their degradation. Thereby pupylation is pivotal for iron homoeostasis and therefore for cell growth. The new insights could help to combat infectious diseases such as tuberculosis. The study published in Proceedings of the National Academy of Sciences USA was selected as "Editor’s Choice" by Science Signaling (Science Signaling: Ferritin out iron with pups).

Research at IBG-1: Biotechnology is focused on Corynebacterium glutamicum already for years. This soil bacterium is used in the industry to produce several million tons of amino acids annually, for example the feed additive L-lysine. It is also a safe model microorganism for the closely related pathogen of tuberculosis, Mycobacterium tuberculosis, both sharing many similarities. One similarity is the gene pup, which encodes for the „prokaryotic ubiquitin-like protein". In M. tuberculosis it is the first component of a complex cell machinery, which first unfolds proteins and then degrades the unfolded proteins for recycling of the protein’s amino acids. The Pup protein is enzymatically covalently linked to target proteins and thereby labels these protein molecules for degradation. The last component of the degradation machinery is the proteasome. In C. glutamicum, however, a proteasome is not present. Well, what then is the task of the pupylation machinery?

To answer this question the pup gene was deleted in the genome of C. glutamicum and the growth of the mutant was monitored under a variety of conditions. In about twenty test series only one condition revealed a growth effect: A limited iron supply resulted in a strongly reduced growth of the genetically modified cells.

Iron is a trace element which is needed by bacteria in relatively high amount. Many essential proteins are iron-dependent and for their functionality iron is required. On the other hand, too much of free iron in a cell together with oxygen is toxic to the cell. Therefore cells harbor proteins which can store free iron, among them ferritin. Complexes of ferritin encapsulate iron in a save form and can provide the iron by release if the cell requires more iron.

Encapsulated iron

In the current study, the bacterial cells without the pup gene exhibited the typical deficiency syndrome as when the supply of iron is limited. Yet the iron content of the cells without the pup gene and that of the wild-type cells having the pup gene was similar! So why is there the iron deficiency syndrome in the pup mutant? Apparently, the iron is still stored by the storage proteins including ferritin, yet iron cannot be released when the Pup protein is absent. The cells have no access to their iron stocks anymore.

The results of our study indicate that the Pup protein plays a pivotal role in the release of iron encapsulated by ferritin. In an earlier study we already showed that in C. glutamicum the Pup protein targets Ferritin (Pupylated proteins in Corynebacterium glutamicum), which is also the case in the pathogen of tuberculosis. Therefore, Pup probably is responsible for the release of iron from ferritin also in the pathogen of tuberculosis. Iron is essential for many bacterial pathogens. Enforced shortage of iron could stop or reduce the growth of pathogens. Therefore, antibiotics targeting the pupylation machinery could help to combat tuberculosis. Furthermore, pupylation could also be interesting for specific applications of C. glutamicum in the industrial biotechnology.

Graphic of Corynebacterium glutamicum Corynebacterium glutamicum is used in the industry to produce several million tons of amino acids annually, for example the feed additive L-lysine. This bacterium is also a safe model microorganism for the closely related pathogen of tuberculosis with which it shares many similarities.
Copyright: Forschungszentrum Jülich

Original publications:

Andreas Küberl, Tino Polen, Michael Bott: The pupylation machinery is involved in iron homeostasis by targeting the iron storage protein ferritin.
Proc Natl Acad Sci USA 113(17):4806-11

Andreas Küberl, Benjamin Fränzel, Lothar Eggeling, Tino Polen, Dirk Wolters, Michael Bott: Pupylated proteins in Corynebacterium glutamicum revealed by MudPIT analysis.
Proteomics 14:1531-1542

Further information:

Science Signaling: Ferritin out iron with pups

Institute of Bio- and Geosciences, Biotechnology (IBG-1)