A conformational switch controlling HIV-1 morphogenesis

Abstract

Assembly of infectious human immunodeficiency virus type 1 (HIV-1) proceeds in two steps. Initially, an immature virus with a spherical capsid shell consisting of uncleaved Gag polyproteins is formed. Extracellular proteolytic maturation causes rearrangement of the inner virion structure, leading to the conical capsid of the infectious virus. Using an in vitro assembly system, we show that the same HIV-1 Gag-derived protein can form spherical particles, virtually indistinguishable from immature HIV-1 capsids, as well as tubular or conical particles, resembling the mature core. The assembly phenotype could be correlated with differential binding of the protein to monoclonal antibodies recognizing epitopes in the HIV-1 capsid protein (CA), suggesting distinct conformations of this domain. Only tubular and conical particles were observed when the protein lacked spacer peptide SP1 at the C-terminus of CA, indicating that SP1 may act as a molecular switch, whose presence determines spherical capsid formation, while its cleavage leads to maturation.

The structural biology of HIV assembly

“…In support of this idea, removal of SP1 can cause recombinant HIV-1 Gag proteins to switch from immaturelike spherical assemblies to mature-like cylinders in vitro [13]. As discussed above, the CA-SP1 region appears to stabilize the immature Gag hexamer, and proteolysis at the CA-SP1 junction therefore presumably destabilizes the Gag lattice.…”

“…To a first approximation, all of the information necessary for retroviral particle assembly resides in the Gag polypeptide. For example, Gag alone can form extracellular virus-like particles in the absence of other viral proteins [11] and Gag molecules can spontaneously assemble into spherical, immature viruslike particles in vitro [12][13][14]. Nevertheless, although Gag itself encodes the necessary tertiary and quaternary interactions, it must be emphasized that assembly requires nonspecific RNA interactions both in vivo and in vitro, and is assisted by host factors in vivo, including trafficking factors, assembly chaperones, and the ESCRT budding pathway, as reviewed elsewhere [15][16][17][18].…”

Cryo-electron microscopy of tubular arrays of HIV-1 Gag resolves structures essential for immature virus assembly

“…1A) (19,20,24), which have a mature-like arrangement of CA. Prior studies had indicated that certain HIV-1 Gag-derived proteins can assemble tubular or spherical particles depending on the conditions (22) and that certain mutations in the CA-SP1 region converted the assembly from spherical to tubular or aberrant sheet-like structures (12,25). Furthermore, mutations in the region targeted by the capsid assembly inhibitor CAI do not alter assembly of immature particles in vitro or in tissue culture, whereas they completely disrupt assembly of the mature-like lattice (26).…”

“…HIV-1 Gag derivatives that assemble tubular arrays adopt a mature-like arrangement of CA (19,20), and have indeed been important tools in determining the structure of the mature CA core (9,21). In contrast, HIV-1 Gag derivatives known to assemble into immature-like arrays form approximately spherical structures (22,23) that are unsuitable for application of high-resolution helical reconstruction methods.…”

In Vitro Identification and Characterization of an Early Complex Linking HIV-1 Genomic RNA Recognition and Pr55Gag Multimerization

“…Taking into account the participation of SP1 in each of the multimerization process (26,30,61), viral maturation (24,25), and recognition of the HIV-1 ⌿ site by NC (31,32), we decided to study relevant RNA-protein interactions in vitro using a minimal Pr55 Gag (mGag) protein. This recombinant protein includes amino acids 2-7 of MA (the myristyl anchor), the CTD of CA, SP1, and NC (28).…”