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 to humans).

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 domains.

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.