P24 Capsid Protein
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The p24 capsid protein is the most abundant HIV protein with each virus containing approximately 1,500 to 3,000 p24 molecules. It is the major structural protein within the
capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or m ...
, and it is involved in maintaining the structural integrity of the virus and facilitating various stages of the viral life cycle, including
viral entry Viral entry is the earliest stage of infection in the viral life cycle, as the virus comes into contact with the host cell (biology), cell and introduces viral material into the cell. The major steps involved in viral entry are shown below. Desp ...
into host cells and the release of new virus particles. Detection of p24 protein's
antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...
can be used to identify the presence of HIV in a person's blood and diagnose HIV/AIDS, however, more modern tests have taken their place. After approximately 50 days of infection, the p24 antigen is often cleared from the bloodstream entirely.


Structure

P24 has a molecular weight of 24 kDa and is encoded by the ''gag'' gene. The structure of HIV capsid was determined by
X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
and
cryo-electron microscopy Cryogenic electron microscopy (cryo-EM) is a transmission electron microscopy technique applied to samples cooled to cryogenic temperatures. For biological specimens, the structure is preserved by embedding in an environment of vitreous ice. An ...
. The p24 capsid protein consists of two domains: the
N-terminal domain The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...
and the C-terminal domain connected by flexible inter-domain linkers. The N-terminal domain (NTD) is made up of 7 α-helices (H) and β-hairpin. The C-terminal domain (CTD) has 4 α-helices and an 11-residue unstructured region. The N-terminal domain (NTD) facilitates contacts within the hexamer, while the C-terminal domain (CTD) forms dimers that bind to adjacent hexamers. Each hexamer contains a size-selective pore surrounded by six positively charged
arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidinium, guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) a ...
residues, and the pore is covered by a β-hairpin that can undergo conformational changes, which has both open and closed conformations. At the center of the hexamers lies an IP6 molecule which stabilizes the tertiary structure of the molecule. Additionally, the C-terminal domain includes a major homology region (MHR) spanning
amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...
153 to 172 with 20 highly conserved amino acids. Moreover, the N-terminal domain features a loop (amino acids 85–93) that interacts with the protein cyclophilin A (Cyp A).


Function

P24 is a structural protein that plays a crucial role in the formation and stability of the viral capsid, which protects the viral
RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...
. p24 capsid protein's roles in the HIV replicative process are summarized as follows: * Fusion: HIV replication cycle begins when HIV fuses with the surface of the host cell. The capsid containing the virus’s genome and proteins then enters the cells. *
Reverse transcription A reverse transcriptase (RT) is an enzyme used to convert RNA genome to DNA, a process termed reverse transcription. Reverse transcriptases are used by viruses such as HIV and hepatitis B virus, hepatitis B to replicate their genomes, by retrot ...
: The capsid ensures the secure transport of the viral genome and reverse-transcription machinery from the cytoplasm's periphery to transcriptionally active sites in the nucleus. It achieves this by shielding the viral genome from detection by restriction factors, while still allowing the necessary molecules to diffuse through the core, facilitating the process of reverse transcription. * Assembly: It is involved in the assembly of new virus particles, facilitating the proper organization of viral components. * Budding: P24 contributes to the viral budding process, ensuring the proper packaging and release of mature and infectious virus particles.


p24 HIV capsid as a therapeutic target


New antiretroviral therapy

Cyclosporine, an immunosuppressant drug designed to prevent organ transplant rejection, has been shown to inhibit infection in HIV-1 positive people. Cyclosporine acts as a competitive inhibitor to the capsid protein’s association with CypA, a cellular protein. CypA has been shown to be important for HIV’s infectivity. The HIV-1 p24 capsid protein plays crucial roles throughout the replication cycle, making it an attractive therapeutic target. Unlike the viral enzymes (
protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...
,
reverse transcriptase A reverse transcriptase (RT) is an enzyme used to convert RNA genome to DNA, a process termed reverse transcription. Reverse transcriptases are used by viruses such as HIV and hepatitis B to replicate their genomes, by retrotransposon mobi ...
and
integrase Retroviral integrase (IN) is an enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme ...
) that are currently targeted by small-molecule antiretroviral drugs, p24 capsid proteins operate through protein-protein interactions. Capsid inhibitors, such as Lenacapavir and GS-6207, interfere with the activities of the HIV capsid protein and underwent evaluation in phase-1 clinical trials as monotherapies. They demonstrated anti-viral activity against all subtypes with no cross-resistance with current antiretroviral drugs. These findings support therapies aimed at disrupting the functions of the HIV capsid protein.


Vaccine design

P24 can induce cellular immune responses and has been included in some vaccine strategies.


Diagnosis


Fourth generation-HIV test

P24 is a target for the immune system, and antibodies against p24 are used in diagnostic tests to detect the presence of HIV antibodies. Fourth-generation HIV immunoassays detect viral p24 protein in the blood and patient antibodies against the virus. Previous generation tests relied on detecting patient antibodies alone; it takes about 3–4 weeks for the earliest antibodies to be detected. The p24 protein can be detected in a patient's blood as early as 2 weeks after infection, further reducing the window period necessary to accurately detect the HIV status of the patient.


See also

* HIV vaccine


References


Further reading

* * {{Viral proteins, state=collapsed Viral structural proteins HIV/AIDS fr:Virus de l'immunodéficience humaine#Structure