Regulation of the nucleic acid chaperone activity of HTLV-1 Nucleocapsid Protein

Abstract

Nucleocapsid proteins (NC) of retroviruses are nucleic acid chaperones that facilitate nucleic acid remodeling. This property of NC proteins is critical for their role in viral genome dimerization, maturation and reverse transcription. In contrast to all other NC proteins studied to date, the human T-cell leukemia virus type 1 (HTLV-1) NC protein was shown to be an extremely poor chaperone. In this work, we demonstrate that the anionic C-terminal domain (CTD) of this protein is responsible for its poor chaperone function. Single molecule DNA stretching studies suggest that HTLV-1 NC dissociates very slowly from single-stranded DNA, which may be a primary reason for its poor chaperone activity. In contrast, a truncation mutant that lacks the CTD is a more effective annealing agent and displays faster off-rate kinetics. Under conditions of high ionic strength, the properties of the WT and CTD-deletion variant are much more similar to each other. Taken together, our data suggest that an electrostatic attraction between the anionic CTD and cationic N-terminal domain of HTLV-1 NC leads to polymerization onto ssDNA resulting in a poor ability to aggregate nucleic acids or to promote their annealing. This property of HTLV-1 NC makes it similar to typical SSB proteins, and may be related to this NC's role in excluding the viral restriction factor APOBEC3G from HTLV particles.