Disparate Contributions of Human Retrovirus Capsid Subdomains to Gag-Gag Oligomerization, Virus Morphology, and Particle Biogenesis

Abstract

The capsid domain (CA) of the retroviral Gag protein is a primary determinant of Gag oligomerization, which is a critical step for immature Gag lattice formation and virus particle budding. Although the human immunodeficiency virus type 1 (HIV-1) CA carboxy-terminal domain (CTD) is essential for CA-CA interactions, the CA CTD has been suggested to be largely dispensable for human T-cell leukemia virus type 1 (HTLV-1) particle biogenesis. To more clearly define the roles of the HTLV-1 CA amino-terminal domain (NTD) and CA CTD in particle biogenesis, we generated and analyzed a panel of Gag proteins with chimeric HIV-1/HTLV-1 CA domains. Subcellular distribution and protein expression levels indicated that Gag proteins with a chimeric HIV-1 CA NTD/HTLV-1 CA CTD did not result in Gag oligomerization regardless of the parent Gag background. Furthermore, chimeric Gag proteins with the HTLV-1 CA NTD produced particles phenotypically similar to HTLV-1 immature particles, highlighting the importance of the HTLV-1 CA NTD in HTLV-1 immature particle morphology. Taken together, these observations support the conclusion that the HTLV-1 CA NTD can functionally replace the HIV-1 CA CTD, but the HIV-1 CA NTD cannot replace the HTLV-1 CA CTD, indicating that the HTLV-1 CA subdomains provide distinct contributions to Gag-Gag oligomerization, particle morphology, and biogenesis. Furthermore, we have shown for the first time that HIV-1 and HTLV-1 Gag domains outside the CA (e.g., matrix and nucleocapsid) impact Gag oligomerization as well as immature particle size and morphology. IMPORTANCE A key aspect in virus replication is virus particle assembly, which is a poorly understood process for most viruses. For retroviruses, the Gag structural protein is the primary driver of virus particle biogenesis, and the CA CTD is the primary determinant of Gag-Gag interactions for HIV-1. In this study, the HTLV-1 capsid amino-terminal domain was found to provide distinct contributions to Gag-Gag oligomerization, particle morphology, and biogenesis. This study provides information that will aid efforts for discovery of therapeutic targets for intervention.

Gag-Gag Interactions Are Insufficient to Fully Stabilize and Order the Immature HIV Gag Lattice

“…Previous comparisons between HIV Gag and other retroviral Gag VLPs have been carried out using both negative staining as well as cryotomography [46,47]. The significant difference observed in our cryotomography between full length immature HIV virions and Gag VLPs were neither explored nor reported before as prior studies had been primarily focused on the shape and curvature of the lattice and not necessary a comparison in ordering of Gag molecules.…”

“…A lattice of Gag molecules lining the inner leaflet of the membrane has been identified in all retroviral genera including alpharetroviruses, betaretroviruses, deltaretroviruses, epsilonretroviruses, gammaretroviruses and lentiviruses (which include HIV) [22,46,48]. Recently, it has also been shown that neuronal gene Arc encodes a Gag protein which forms a lattice in neuronal cells and mediates RNA transfer [49].…”

Reviewing HIV-1 Gag Mutations in Protease Inhibitors Resistance: Insights for Possible Novel Gag Inhibitor Designs

“…Such a “wrench” could be linked with peptide inhibitors devised from the third approach of linking two inhibitors to bind at least two conserved sites in Gag to constrain Gag conformation. Potential binding sites can be the conserved regions of NC and p6 to rigidify p6 and perturb the allosteric signaling to the MA–CA region [60] to also interfere with required Gag oligomerization [91,92,93]. By preventing Gag conformational change from the compact to the extended structure, the exposure of subsequent cleavage sites could be reduced, also impairing the viral fitness by slowing down the viral maturation process.…”

Structure, Function, and Interactions of the HIV-1 Capsid Protein

“…Disruption of the Gag-filamin A interaction eliminates localization and accumulation of Gag at the plasma membrane [ 99 ]. Once assembled, the CA and SP1 domains are responsible for stabilizing the Gag-Gag interactions that create the oligomeric lattice of the immature virion [ 100 , 101 , 102 , 103 ]. The immature arrangement of CA differs greatly from the mature arrangement of the capsid core.…”