Immune defense mechanism: How proteins bring together membrane blebs

The team led by Prof Dr Christian Herrmann and Dr Sergii Shydlovskyi from the Bochum cluster of excellence Resolv and Dr Gerrit Praefcke, formerly of the University of Cologne, now at the Paul-Ehrlich-Institut in Langen, reports on the study in the journal Proceedings of the National Academy of Sciences, PNAS for short.

Precursor of vesicle fusion

With a combination of cell biology and biochemical experiments, the researchers explored the function of human guanylate-binding protein 1 (hGBP1). In cells, it interacts with the energy storage molecule GTP, from which it can split off one or two phosphate groups, in order to release energy.

In the current study, the researchers discovered that hGBP1 uses energy released during splitting to change its structure: it unveils a lipid anchor. Using this anchor, it can form larger ring-shaped polymers with other hGBP1 proteins. With the aid of artificial vesicles, the team also found that hGBP1 uses the anchor to bind to the vesicle membrane. In this way, it brings together many such membrane blebs, which the researchers assume could be a precursor to vesicle fusion.

Demonstrated in cells

This kind of fusion is crucial for the immune defence mechanism: pathogens are trapped in the human body in vesicles, which merge with certain cell organelles, lysosomes. The latter contain enzymes that degrade pathogens. In the current study, the team also demonstrated that the protein hGBP1 in living cells is actually involved in the signal path, which leads via the lysosomes to the degradation of viruses and bacteria.