B02: Subversion of host cell vesicle trafficking: Hijacking of autophagy-related proteins by HIV-1 Nef

The human immunodeficiency virus type 1 (HIV-1) has developed specific mechanisms to escape the host organism antiviral defence and even to misuse it for its own benefit. Some of these antiviral defence strategies use autophagy e.g. for the presentation of antigens or the degradation of the pathogens. In general, autophagy is a highly conserved process in eukaryotes to degrade cellular compounds. Autophagy plays an important role for basic turn-over of intracellular proteins and organelles as well as for the production of amino acids under starvation conditions. During autophagy the proteins, organelles or pathogens are engulfed by double-membrane structures, called phagophores. Under normal conditions, the phagophore closes to form the autophagosome, which fuses with the lysosome leading to the degradation of its content. Several autophagy-related proteins (ATGs) are needed for this process. Among them are ATG8s. These proteins can be posttranslational lipidated and are important for the elongation and closure of the phagophore. Whereas yeast only codes for one Atg8, there are at least seven different ATG8 proteins in humans, which are divided in two subfamilies, the LC3s (microtubule-associated protein 1 light chain 3) and the GABARAPs (γ-amino-butyric acid receptor-associated protein). Whereas the LC3s, especially LC3B, are important for the phagophore elongation and the GABARAPs are thought to be essential for the phagophore closure, the exact functions of the individual proteins are still unknown.

Currently, people are beginning to uncover the impact of ATGs during unconventional protein secretion (UPS). It was shown in yeast that the formation of compartments of unconventional protein secretion (CUPS) is induced in a manner similar to autophagy e.g. by starvation and that the CUPS are Atg8 positive. Further, in mammalians it was shown for interleukin-β1, that this protein is secreted utilizing LC3B-positive autophagosome-like vesicles as intermediates. As the pathway described in yeast utilizes single-lipid bilayer vesicles, we termed this process “sUPS” for single-lipid bilayer vesicle-based unconventional protein secretion. The second process uses double-lipid bilayer vesicles. Thus, for the present project description, we termed it “dUPS” for double-lipid bilayer vesicle-based unconventional protein secretion. Here, we will investigate the function of the HIV‑1 Nef protein in modulating cellular vesicle trafficking including autophagosome-like structures and exosomes, which are known to be released in high numbers in the presence of this viral accessory protein. Nef is a protein that mediates its pleiotropic functions through interactions with host cell proteins. One example is the inhibition of autophagy via interaction with the autophagy regulator Beclin1. To fulfil several of its functions Nef has to be transported anterograde towards the plasma membrane (PM) or secreted. However, Nef does not contain a signal peptide, excluding a conventional protein secretion pathway. Thus, the molecular mechanism underlying Nef’s transport is yet unknown. Own preliminary data demonstrate that Nef undergoes direct interaction with several members of the human ATG8 family and that way Nef might orchestrate the fate of intracellular as well as extracellular vesicles, and finally its own secretion. The goal of this project is to test the hypothesis that hijacking cellular ATG8 protein family members by HIV-1 Nef might be a yet unrecognised mechanism of increasing Nef’s own secretion and finally enhancing e.g. HIV-1’s viral persistence, infectivity, and pathogenesis by subverting the flux of host membrane systems during viral infection. Further, this way of unconventional protein secretion might be a general secretion mechanism that is used by other proteins lacking a signal peptide sequence. Thus, we will on the one hand learn how the viral protein interferes with cellular trafficking pathways leading to its secretion, but also learn about general mechanisms for unconventional protein secretion. On the other hand we will gain insights into the functions and functional differences between the different human ATG8 proteins in general.

Project leader:
Prof. Dr. Dieter Willbold, undefined email,
                        undefined Institute of Physical Biology

Researchers: Serife Akgülundefined email
                        Julia Sanwald, undefined email
                        Indra Simons, undefined email

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