<p>Gag itself is sufficient for immature-particle assembly. The interactions critical for immature-particle assembly are mediated by CA-CA and NC-RNA interfaces. CA consists of two subdomains, the CA&ndash;N-terminal domain (NTD) and the CA&ndash;C-terminal domain (CTD), connected by a short sequence. Despite a low level of sequence homology, all available retroviral CA-NTD structures show conserved structural motifs consisting of six or seven &alpha;-helices and an N-terminal &beta;-hairpin anchored by a salt bridge between the N-terminal proline and an aspartate in helix 3.</p>
<p>The assembly of fully infectious retroviruses comprises two different phases: immature- and mature-particle formation. Initially, the virus is formed in immature form by self-association of the major structural polyprotein Gag into spherical particles either at the plasma membrane (alpharetroviruses, gammaretroviruses, or lentiviruses) or at a distinct site within the infected cell (betaretroviruses). The particles are organized by lattices of Gag and Gag-derived polyproteins that associate with the plasma membrane that becomes the outer layer of the immature particle released during budding. Upon release from the host cell, the Gag precursor is proteolytically processed into N-terminal membrane binding matrix protein (MA), two-domain capsid protein (CA), and RNA-binding nucleocapsid protein (NC). This cleavage leads to a dramatic morphological change and reorganization of the virus particle from its immature form into a mature, infectious virion.</p>