Nef (protein)

Nef (Negative Regulatory Factor) is a small 27-35 kDa myristoylated protein encoded by primate lentiviruses. These include Human Immunodeficiency Viruses (HIV-1 and HIV-2) and Simian Immunodeficiency Virus (SIV). Nef localizes primarily to the cytoplasm but also partially to the Plasma membrane (PM) and is one of many pathogen-expressed proteins, known as virulence factors, which function to manipulate the host's cellular machinery and thus allow infection, survival or replication of the pathogen.[1] Nef stands for "Negative Factor" and although it is often considered dispensable for HIV-1 replication, in infected hosts the viral protein markedly elevates viral titers.[2]

Negative factor, (F-Protein) or Nef
NMR structure of the NEF protein based on the PDB: 2NEF​ coordinates.
Identifiers
SymbolF-protein
PfamPF00469
InterProIPR001558
SCOP21avv / SCOPe / SUPFAM
OPM superfamily269
OPM protein2nef

Function

The expression of Nef early in the viral life cycle ensures T-cell activation and the establishment of a persistent state of infection, two basic attributes of HIV infection. Viral expression of Nef induces numerous changes within the infected cell including the modulation of protein cell surface expression, cytoskeletal remodeling, and signal transduction. Since the activation state of the infected cell plays an important role in the success rate of HIV-1 infection, it is important that resting T-cells be primed to respond to T-cell receptor (TCR) stimuli. HIV-1 Nef lowers the threshold for activation of CD4+ lymphocytes, but is not sufficient to cause activation in the absence of exogenous stimuli.[3]

By down regulating cell surface expression of CD4 and Lck, Nef creates a narrow TCR response which likely optimizes HIV-1 viral production and generates a susceptible population of cells to further infect. Nef retargets kinase-active Lck away from the plasma membrane to early and recycling endosomes (RE) as well as the Trans-Golgi network (TGN). RE/TGN associated Lck sub-populations in Nef expressing cells are in the catalytically active conformation and thus signaling competent.[4] While TCR signaling takes place at the plasma membrane (PM), activation of the Ras-GTPase takes place in intracellular compartments including the Golgi apparatus. Nef induced enrichment of active Lck in these compartments results in an increase of localized RAS activity and enhanced activation of Erk kinase and the production of Interleukin-2 (IL-2).[5] Since IL-2 is known to activate the growth, proliferation, and differentiation of T-cells to become effector T-cells; this is a self-serving effect that creates a new population of cells which HIV-1 is able to infect. Self-activation of the infected cell by IL-2 also stimulates the cell to become an effector cell and initiate the machinery which HIV-1 relies upon for its own proliferation.

To further evade the host immune response, Nef down-regulates the cell surface and total expression of the negative immune modulator CTLA-4 by targeting the protein for lysosomal degradation. In contrast to CD28 which activates T-cells, CTLA-4 is essentially an “off-switch” which would inhibit the viral production if it were activated. Lentiviruses such as HIV-1 have acquired proteins such as Nef which perform a wide array of functions including the identification of CTLA-4 before it reaches the PM and tagging it for degradation.[6] Nef is also known to phosphorylate and inactivate Bad, a proapoptotic member of the Bcl-2 family thus protecting the infected cells from apoptosis.

Cytoskeletal remodeling is thought to reduce TCR signaling during early infection and is also modulated to some degree by Nef. Actin remodeling is generally modulated by the actin severing factor cofilin. Nef is able to associate with the cellular kinase PAK2 which phosphorylates and inactivates cofilin and interferes with early TCR signaling.

Clinical significance

The Sydney blood bank cohort (SBBC) were a group of eight patients who were asymptomatic many years after initial infection by transfusion from an infected blood donor. Later analyses showed that the virus strain was a Nef-deleted variant.[7]

Vaccine

A Nef-deleted virus vaccine has not been tried in humans although it was successfully tested in Rhesus macaques.[8][9]

See also

References
  1. Das SR, Jameel S (April 2005). "Biology of the HIV Nef protein" (PDF). Indian J. Med. Res. 121 (4): 315–32. PMID 15817946.
  2. Marcey D, Somple M, Silva N (2007-01-01). "HIV-1 Nef Protein". The Online Macromolecular Museum Exhibits. California Lutheran University. Retrieved 2008-08-06.
  3. Abraham L, Fackler OT (December 2012). "HIV-1 Nef: a multifaceted modulator of T cell receptor signaling". Cell Communication and Signaling. 10 (1): 39. doi:10.1186/1478-811X-10-39. PMC 3534016. PMID 23227982.
  4. Laguette N, Bregnard C, Benichou S, Basmaciogullari S (June 2010). "Human immunodeficiency virus (HIV) type-1, HIV-2 and simian immunodeficiency virus Nef proteins". Mol. Aspects Med. 31 (5): 418–33. doi:10.1016/j.mam.2010.05.003. PMID 20594957.
  5. Geyer M, Fackler OT, Peterlin BM (July 2001). "Structure–function relationships in HIV-1 Nef". EMBO Reports. 2 (7): 580–5. doi:10.1093/embo-reports/kve141. PMC 1083955. PMID 11463741.
  6. El-Far M, Isabelle C, Chomont N, Bourbonnière M, Fonseca S, Ancuta P, Peretz Y, Chouikh Y, Halwani R, Schwartz O, Madrenas J, Freeman GJ, Routy JP, Haddad EK, Sékaly RP (January 2013). "Down-Regulation of CTLA-4 by HIV-1 Nef Protein". PLOS ONE. 8 (1): e54295. Bibcode:2013PLoSO...854295E. doi:10.1371/journal.pone.0054295. PMC 3553160. PMID 23372701.
  7. Learmont JC, Geczy AF, Mills J, Ashton LJ, Raynes-Greenow CH, Garsia RJ, Dyer WB, McIntyre L, Oelrichs RB, Rhodes DI, Deacon NJ, Sullivan JS (June 1999). "Immunologic and virologic status after 14 to 18 years of infection with an attenuated strain of HIV-1. A report from the Sydney Blood Bank Cohort". N. Engl. J. Med. 340 (22): 1715–22. doi:10.1056/NEJM199906033402203. PMID 10352163.
  8. Daniel MD, Kirchhoff F, Czajak SC, Sehgal PK, Desrosiers RC (December 1992). "Protective effects of a live attenuated SIV vaccine with a deletion in the nef gene". Science. 258 (5090): 1938–41. Bibcode:1992Sci...258.1938D. doi:10.1126/science.1470917. PMID 1470917.
  9. Muthumani K, Choo AY, Hwang DS, Premkumar A, Dayes NS, Harris C, Green DR, Wadsworth SA, Siekierka JJ, Weiner DB (September 2005). "HIV-1 Nef-induced FasL induction and bystander killing requires p38 MAPK activation". Blood. 106 (6): 2059–68. doi:10.1182/blood-2005-03-0932. PMC 1895138. PMID 15928037.

Further reading
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