<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>