<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>