3D). Detailed three-dimensional (3D) structural analysis of these particles revealed that below a disordered N-terminal CA layer, the C terminus of CA assembles a typical immature lattice, which is linked by rod-like densities with the RNP. == INTRODUCTION == 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 LY500307 distinct site within the infected cell (betaretroviruses). The particles are organized by lattices of Gag and Gag-derived LY500307 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. Gag itself is sufficient for immature-particle LY500307 assembly. The interactions critical for immature-particle assembly are mediated by CA-CA and NC-RNA interfaces. CA consists of two subdomains, the CAN-terminal domain (NTD) and the CAC-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 -helices and an N-terminal -hairpin anchored by a salt bridge between the N-terminal proline and an aspartate in helix 3 (10,20,29,30,4244). CA-CTD contains a dimerization domain and is essential for the assembly of Gag. In the alpharetroviruses (e.g., Rous sarcoma virus LY500307 [RSV]) and lentiviruses (e.g., HIV-1), a short spacer peptide (SP) (12 or 14 amino acids, respectively) separates CA from NC. This SP domain plays a critical role in assembly, as mutations or deletions in the SP influence immature-spherical-particle assembly (1,28,33,37). Both HIV-1 and RSV SP sequences were also suggested to function as molecular switches, because the deletion of SP1 of HIV-1 Gag led to the formation of tubular instead of spherical particles (23). Similarly, mutations within the SP sequence of RSV Gag change spherical-particle formation to tubular-particle formation (28). The appearances of all immature retroviral particles observed by transmission electron microscopy (TEM) are similar; they are roughly spherical, with an inner electron-dense ring. Based on cryo-electron microscopy (cEM) studies, the Gag polyprotein is arranged radially and is associated with the plasma membrane through its MA domain, with the NC domain oriented toward the center of the particle (65,67). Three-dimensional (3D) reconstructions of immature HIV-1 and RSV particles obtained LY500307 by cryo-electron tomography have shown that the immature Gag lattice adopts a hexagonal arrangement (5,66). To form a closed sphere from a hexagonal lattice, the incorporation of pentamers or irregular defects is required. Pentamers have not been observed in the immature Gag lattice; instead, the Gag hexameric lattice closes through the LIG4 incorporation of heterogeneously shaped defects (6,35). Upon maturation, NC with bound RNA condenses in the center of the particle, surrounded by a core shell assembled from released CA. The shapes of the mature cores differ according to the retrovirus genus: in gamma-, delta-, and alpharetroviruses, they are spherical; in betaretroviruses, cylindrical; and in lentiviruses, conical. These assemblies are composed of a curved hexagonal lattice closed by incorporation of CA pentamers. The final shapes of the retroviral cores are then determined by the locations of the pentamers. The core of the most studied retrovirus, HIV-1, is a.