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Investigating the molecular basis of the bacterial endocytosis-like protein uptake

Subject Area Metabolism, Biochemistry and Genetics of Microorganisms
Microbial Ecology and Applied Microbiology
Term from 2013 to 2018
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 239740592
 
Final Report Year 2018

Final Report Abstract

In summary the projects revolutionized the concept of the planctomycetal cell plan. Initially, we aimed to reveal the molecular mechanism of endocytosis-like protein uptake in Planctomycetes. However, we soon realized that there was no endocytosis-like uptake and that high molecular weight substrates were not taken up into vesicles but stored in the enlarged periplasmic space instead. Planctomycetes were believed to lack the canonical Gram- negative outer membrane and a peptidoglycan (PG) cell wall. We were able to show that instead Planctomycetes have both, a PG cell wall and a Gram-negative outer membrane. However, this more typical Gram-negative cell plan is counterintuitive to endocytosis which requires inward vesicle formation at the outermost membrane which is prevented by lipopolysaccharides in Gramnegative outer membranes. Thus, uptake must employ another mechanism than endocytosis. However, canonical OMP transport channels are too small to facilitate uptake of complex substrates as used in our study. Thus, a novel unknown mechanism must exist that allows Planctomycetes the fast incorporation of such high molecular weight substrates. We demonstrated evidence that instead of endocytosis, a novel fiber mediated uptake mechanism might be employed. Planctomycetes possess unique donut shaped ring structures within their outer membranes. Such structures are unique among bacteria, and we demonstrated that fiber-like structures originate from crateriform structures. Employing gold labeling of high molecular weight substrates, we further proved that such substrates bind to fibers. We raised the hypothesis that such fibers act as ‘molecular fishing rod’ to harvest for example complex carbon substrates into the periplasmic space where they are subject to degradation. This ‘selfish feeding’ mechanism has the great advantage that multiple enzymes can be accumulated in the enlarged periplasmic space that perform the degradation. Other bacteria secrete such substrates into the environment, a strategy prone to cheating by other, non-enzyme- producing species. Taken together, the projects were very successful and significantly changed our picture of Planctomycetes. The genetic tools developed by Damien Devos’s group will be very instrumental for future in-depth analysis of this novel fiber mediated uptake mechanism.

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