The capsid protein of human immunodeficiency virus: intersubunit interactions during virus assembly

The capsid protein (CA) of HIV-1 is composed of two domains, the N-terminal domain (NTD) and the C-terminal domain (CTD). During the assembly of the immature HIV-1 particle, both CA domains constitute a part of the Gag polyprotein, which forms a spherical capsid comprising up to 5000 radially arranged, extended subunits. Gag-Gag interactions in the immature capsid are mediated in large part by interactions between CA domains, which are involved in the formation of a lattice of connected Gag hexamers. After Gag proteolysis during virus maturation, the CA protein is released, and approximately 1000-1500 free CA subunits self-assemble into a truncated cone-shaped capsid. In the mature capsid, NTD-NTD and NTD-CTD interfaces are involved in the formation of CA hexamers, and CTD-CTD interfaces connect neighboring hexamers through homodimerization. The CA-CA interfaces involved in the assembly of the immature capsid and those forming the mature capsid are different, at least in part. CA appears to have evolved an extraordinary conformational plasticity, which allows the creation of multiple CA-CA interfaces and the occurrence of CA conformational switches. This minireview focuses on recent structure-function studies of the diverse CA-CA interactions and interfaces involved in HIV-1 assembly. Those studies are leading to a better understanding of molecular recognition events during virus morphogenesis, and are also relevant for the development of anti-HIV drugs that are able to interfere with capsid assembly or disassembly.