HIV-1 gag Protein Fragments

The gag gene encodes the Gag precursor protein of 55kD, also known as P55 protein. When Gag is translated and expressed, the N-terminal is modified by cardamom acylation, which promotes its binding to the cytoplasmic side of the cell membrane. The Gag polyprotein bound to the membrane recruits two copies of the viral genomic RNA, together with other viruses and cell proteins, to trigger viral particles to sprout and secrete from the infected cell surface. After budding, Gag is cut by virus-encoded proteases into four smaller proteins: MA (matrix proteins, also known as p17), CA (capsid protein, also known as p24), NC (nucleocapsid protein, also known as p9) and p6.

Four smaller proteins

1. MA (matrix proteins, also known as p17): The polyprotein is targeted at the plasma membrane through multicomponent membrane binding signals, including its myristoylated N-terminal. Matrix protein is part of the pre-integration complex. It is related to the release of host cells mediated by Vpu.
2. CA (capsid protein, also known as p24): CA protein forms the core of virus particles. Intracellular cyclophilin A can bind to the CA protein region on Gag, and blocking this binding can inhibit virus replication, indicating that their binding is very important for virus replication. Most of the cores are conical, with only 7% of the tubes. The core is composed of capsid protein hexamer subunits. Shortly after the virus enters, the core of the virus is dismantled. The binding of host limiting factors (such as TRIM5- α or TRIMCyp) to retrovirus capsid leads to the premature disintegration of the viral capsid, which leads to the blocking of reverse transcriptase. The restriction of TRIM5 on HIV-1 capsid is one of the factors limiting HIV-1 infection in humans.
3. NC (nucleocapsid protein, also known as p9): The NC region of Gag can specifically recognize the packaging signal of HIV. This packaging signal is composed of four stem-loop structures at the 5' end of the virus RNA, which can effectively mediate the heterologous RNA into the virus particles of HIV. NC protein can also promote the reverse transcription of the virus.
4. The p6 region of Gag mediates the binding of Gag to the helper protein Vpr, allowing Vpr to enter the assembled viral plasmid. The P6 protein also contains a so-called late region, which is necessary for the budding process of the virus.

Conclusion

Human immunodeficiency virus (HIV-1) is a retrovirus that can cause acquired immunodeficiency syndrome (AIDS), which is a highly fatal infectious disease. Gag protein is not only the main structural protein of HIV-1, but also the necessary structural protein for HIV-1 spherical capsid assembly. The in-depth study of the transport, polymerization and assembly of Gag protein is helpful to further understand the replication mechanism of HIV-1, and accelerate the development of new and efficient antiviral drugs.

References

1. Nixon, D. F., Townsend, A. R., Elvin, J. G., Rizza, C. R., Gallwey, J., & McMichael, A. J. (1988). HIV-1 gag-specific cytotoxic T lymphocytes defined with recombinant vaccinia virus and synthetic peptides. Nature, 336(6198), 484.
2. Chang, N. T., Gallo, R. C., & Wong-Staal, F. (2016). U.S. Patent No. 9,328,391. Washington, DC: U.S. Patent and Trademark Office.