Critical Role of the Human T-Cell Leukemia Virus Type 1 Capsid N-Terminal Domain for Gag-Gag Interactions and Virus Particle Assembly

J Virol. 2018 Jun 29;92(14):e00333-18. doi: 10.1128/JVI.00333-18. Print 2018 Jul 15.


The retroviral Gag protein is the main structural protein responsible for virus particle assembly and release. Like human immunodeficiency virus type 1 (HIV-1) Gag, human T-cell leukemia virus type 1 (HTLV-1) has a structurally conserved capsid (CA) domain, including a β-hairpin turn and a centralized coiled-coil-like structure of six α helices in the CA amino-terminal domain (NTD), as well as four α-helices in the CA carboxy-terminal domain (CTD). CA drives Gag oligomerization, which is critical for both immature Gag lattice formation and particle production. The HIV-1 CA CTD has previously been shown to be a primary determinant for CA-CA interactions, and while both the HTLV-1 CA NTD and CTD have been implicated in Gag-Gag interactions, our recent observations have implicated the HTLV-1 CA NTD as encoding key determinants that dictate particle morphology. Here, we have conducted alanine-scanning mutagenesis in the HTLV-1 CA NTD nucleotide-encoding sequences spanning the loop regions and amino acids at the beginning and ends of α-helices due to their structural dissimilarity from the HIV-1 CA NTD structure. We analyzed both Gag subcellular distribution and efficiency of particle production for these mutants. We discovered several important residues (i.e., M17, Q47/F48, and Y61). Modeling implicated that these residues reside at the dimer interface (i.e., M17 and Y61) or at the trimer interface (i.e., Q47/F48). Taken together, these observations highlight the critical role of the HTLV-1 CA NTD in Gag-Gag interactions and particle assembly, which is, to the best of our knowledge, in contrast to HIV-1 and other retroviruses.IMPORTANCE Retrovirus particle assembly and release from infected cells is driven by the Gag structural protein. Gag-Gag interactions, which form an oligomeric lattice structure at a particle budding site, are essential to the biogenesis of an infectious virus particle. The CA domain of Gag is generally thought to possess the key determinants for Gag-Gag interactions, and the present study has discovered several critical amino acid residues in the CA domain of HTLV-1 Gag, an important cancer-causing human retrovirus, which are distinct from that of HIV-1 as well as other retroviruses studied to date. Altogether, our results provide important new insights into a poorly understood aspect of HTLV-1 replication that significantly enhances our understanding of the molecular nature of Gag-Gag interaction determinants crucial for virus particle assembly.

Keywords: Gag; deltaretrovirus; lentiviruses; morphology; oligomerization; retrovirus; virus assembly.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Capsid / chemistry
  • Capsid / metabolism*
  • Capsid Proteins / chemistry
  • Capsid Proteins / metabolism*
  • Gene Products, gag / chemistry*
  • Gene Products, gag / genetics
  • Gene Products, gag / metabolism*
  • HTLV-I Infections / metabolism
  • HTLV-I Infections / virology*
  • HeLa Cells
  • Human T-lymphotropic virus 1 / physiology
  • Humans
  • Models, Molecular
  • Mutation
  • Protein Domains
  • Virion / pathogenicity*
  • Virus Assembly*
  • gag Gene Products, Human Immunodeficiency Virus


  • Capsid Proteins
  • Gene Products, gag
  • gag Gene Products, Human Immunodeficiency Virus
  • p19 protein, Human T-lymphotropic virus 1