HIV-1 gag (group specific antigen) encodes a 53 kDa
polyprotein, which consists of proteins that mediate virion
assembly and budding (binding plasma membrane, creating necessary
protein-protein interactions to create spherical particles,
recruiting envelope proteins and RNA genome packaging). Translation
of gag occurs on the full unspliced mRNA. The polyprotein
will form spherical arrays inside the budding immature virus, which
will then undergo autocatalytic cleavage into four proteins: p17,
p24, p7 and p6.
Matrix (MA) p17 targets the polyprotein to the plasma membrane and
is part of the pre-integration complex. This protein is implicated
in the release from host cell and binds RNA. Post-cleavage from
gag, the protein is found underlying the envelope. MA consists of
128 amino acids.
Capsid (CA) protein p24 forms the structural, conical core that
encapsulates the genome RNA-nucleocapsid complex in the virion. The
core is constituted by hexamer subunits and is disassembled after
virion entry. Formation of mature virions is dependent on CA. In
other non-human species (such as monkeys), host factors bind this
protein to cause premature capsid disassembly (thus restricting HIV
Nucleocapsid (NC) protein p7 encapsulates and protects viral
dimeric unspliced genome RNA. This protein acts a nucleic acid
chaperone that causes a structural conformational change to allow
for genome RNA retrotranscription. As part of the polyprotein, it
functions in genomic RNA dimerization, packaging, tRNA
incorporation and virion assembly (structural role in the immature
virion). NC is 55 amino acids long and contains zinc finger
p6 is the C-terminus of Gag and is 52 amino acids long. It is
important in budding of the assembled particle.
Spacer proteins p2 (SP1; 14 amino acids) and p1 (SP2; 16 amino
acids) are also present between CA + NC and NC + p6, respectively.