Perturbation of human T-cell leukemia virus type 1 particle morphology by differential gag co-packaging

José O. Maldonado, Isaac Angert, Sheng Cao, Serkan Berk, Wei Zhang, Joachim D. Mueller, Louis M. Mansky

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


Human T-cell leukemia virus type 1 (HTLV-1) is an important cancer-causing human retrovirus that has infected approximately 15 million individuals worldwide. Many aspects of HTLV-1 replication, including virus particle structure and assembly, are poorly understood. Group-specific antigen (Gag) proteins labeled at the carboxy terminus with a fluorophore protein have been used extensively as a surrogate for fluorescence studies of retroviral assembly. How these tags affect Gag stoichiometry and particle morphology has not been reported in detail. In this study, we used an HTLV-1 Gag expression construct with the yellow fluorescence protein (YFP) fused to the carboxy-terminus as a surrogate for the HTLV-1 Gag-Pol to assess the effects of co-packaging of Gag and a Gag-YFP on virus-like particle (VLP) morphology and analyzed particles by cryogenic transmission electron microscopy (cryo-TEM). Scanning transmission electron microscopy (STEM) and fluorescence fluctuation spectroscopy (FFS) were also used to determine the Gag stoichiometry. We found that ratios of 3:1 (Gag:Gag-YFP) or greater resulted in a particle morphology indistinguishable from that of VLPs produced with the untagged HTLV-1 Gag, i.e., a mean diameter of ~113 nm and a mass of 220 MDa as determined by cryo-TEM and STEM, respectively. Furthermore, FFS analysis indicated that HTLV-1 Gag-YFP was incorporated into VLPs in a predictable manner at the 3:1 Gag:Gag-YFP ratio. Both STEM and FFS analyses found that the Gag copy number in VLPs produced with a 3:1 ratio of Gag:Gag-YFP was is in the range of 1500-2000 molecules per VLP. The observations made in this study indicate that biologically relevant Gag-Gag interactions occur between Gag and Gag-YFP at ratios of 3:1 or higher and create a Gag lattice structure in VLPs that is morphologically indistinguishable from that of VLPs produced with just untagged Gag. This information is useful for the quantitative analysis of Gag-Gag interactions that occur during virus particle assembly and in released immature particles.

Original languageEnglish (US)
Article number191
Issue number7
StatePublished - Jul 19 2017

Bibliographical note

Publisher Copyright:
© 2017 by the authors. Licensee MDPI, Basel, Switzerland.


  • Cryogenic transmission electron microscopy (cryo-TEM)
  • Fluorescence fluctuation spectroscopy (FFS)
  • Gag stoichiometry
  • Human T-cell leukemia virus type 1 (HTLV-1)
  • Scanning transmission electron microscopy (STEM)


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