The HIV Protein NEF on the Trail

Jülich researchers are discovering the important role of the previously unknown protein

HIV protein
Figure left: HIV-1 NEF uses the already identified hydrophobic pockets 1 and 2 for binding to GABARAP. In the figure, the amino acids relevant for the binding of GABARAP were stained. Residues strongly influenced by the binding are shown in yellow in lilac colors and residues which are unaffected by the interaction. Figure on the right: fluorescence micrograph of an HIV-1 NEF-expressing cell. HIV-1 NEF (red) is located on the plasmic membrane, near the cell nucleus, and in vesicular structures on the plasmic membrane due to its many sites. Copyright: Research Center Jülich / Heinrich-Heine-University Düsseldorf

Around 37 million people worldwide live with HIV infection. So far, the associated disease, the acquired immunodeficiency syndrome (AIDS), can only be treated with medicines – but not healed. The human immunodeficiency virus remains one of the biggest challenges for the public health system. The viral protein NEF (Negative Factor) is considered an important virulence factor, which contributes decisively to the course of the disease.Scientists at the Jülich Institute of Complex Systems and at the Düsseldorf Institute of Physical Biology have now discovered that NEF needs a certain interaction partner to get to the plasmamembrane of the host cell: the so-called GABA (A) receptor-associated protein (GABARAP). If the researchers switch off the protein, NEF no longer reaches the inside of the plasma membrane. The article published in the scientific journal Scientific Reports helps to understand both the functions of the viral and the human protein better.

For a long time, NEF was considered unimportant for HIV disease, hence its name: negative factor; A small viral protein without enzymatic function, which is dependent on the virus isolate from 200 to 280 amino acids and belongs to the less than 20 HI virus proteins. However, it is now known that NEF is one of the most important protein components of the HI virus. It manipulates human immune cells in a variety of ways and thereby contributes to the effective spread and high virulence of HIV in humans.

In order for NEF to perform some of its functions, it must reach the inside of the cell surface of the infected cell. How this is done to him efficiently, which mechanisms are necessary, is so far unknown. The Jülich working group around Silke Hoffmann of the Institute of Complex Systems, Department of Structural Biochemistry (ICS-6) has now demonstrated in human cell systems that NEF requires the host protein GABARAP or its close relatives GABARAPL1 and -L2 to dock on the cell membrane.”It is only when NEF is localized to the plasma membrane that it can initiate the down-regulation of the immune system, for example, by the removal of important surface receptors such as CD4 or MHC-I,” says Hoffmann.

The importance of NEF for the course of the disease is also reflected in those patients infected with virus isolates with the NEF protein defective: “These people can carry the HI virus for a long time without the disease erupting” , Explains Hoffmann. Therefore, there is great interest in science to decipher the various NEF functions – a huge puzzle. “We were all the more astonished to find a human interaction partner with GABARAP, a protein that has been studied at our institute for many years at a structural biology level, which we can say: if we take it out of our cell lines, NEF no longer reaches it “Said Alexandra Boeske, who has carried out the crucial experiments on the ICS-6 and is currently working at the Heinrich Heine University in Düsseldorf (HHU). Against the background that the localization of NEF on the plasma membrane is so important for many effects of HIV pathogenesis, a decisive gain in knowledge.

The scientists were able to identify the binding site for HIV-NEF on the molecular surface of GABARAP at the biomolecular NMR center in Jülich, the corresponding 3D structure of GABARAP had been deciphered earlier. In the next step, in cooperation with virologists, the researchers will investigate whether infected cells in the absence of GABARAP actually have no down-regulation of immunoreceptors and thus of the immune system. “This would prove our supposed importance of GABARAP in the process in the biologically relevant system,” says Hoffmann.

GABARAP itself belongs to a protein family that is involved in intracellular transport processes as well as in autophagy, ie the process in which cells separate and recycle their own components, but also use them to fight viruses and bacteria.Moreover, cells appear to use at least parts of the autophagic process for the secretion of proteins. To match this, the researchers have observed that the absence of GABARAP not only prevents NEF from being localized to the cell membranes, but also transports the viral protein to the outside into the extracellular space, where NEF is known to damage noninfected cells and thus also crucial to pathogenesis contributes. The exact relationship between autophagy, GABARAP and NEF secretion will be promptly presented by the structural and molecular biologists in a further publication. The project is also funded by the DFG within the framework of the SFB1208 special research area.

Source : Forschungszentrum Jülich