HIV-1, like other retroviruses, expresses its protein genes by first generating a polyprotein, consisting of head-to-tail assembly of all (or nearly all) HIV-1 proteins. These polyproteins must undergo proteolysis at specific sites to generate the individual proteins that are used in HIV replication, packaging, and egress from infected cells. All known retroviruses encode a protease that performs this function, and HIV-1 protease specifically chops up the HIV-1 polyprotein into virus-specific functional proteins (Dunn et al., 2002).
Dunn, Goodenow, Gustchina & Wlodawer. 2002. Genome Biology. Protein Family Review: Retroviral proteases.
Kuriyan, Konforti & Wemmer. 2008. The Molecules of Life: Physical and Chemical Principles.