HIV-1 Gag polyprotein orchestrates the assembly of viral particles. Its C-terminus consists of the nucleocapsid (NC) domain that interacts with nucleic acids, and p1 and p6, two unstructured regions, p6 containing the motifs to bind ALIX, the cellular ESCRT factor TSG101 and the viral protein Vpr. The processing of Gag by the viral protease subsequently liberates NCp15 (NC-p1-p6), NCp9 (NC-p1) and NCp7, NCp7 displaying the optimal chaperone activity of nucleic acids. This review focuses on the nucleic acid binding properties of the NC domain in the different maturation states during the HIV-1 viral cycle.

In free B-DNA, slow (microsecond-to-millisecond) motions that involve equilibrium between Watson–Crick (WC) and Hoogsteen (HG) base-pairing expand the DNA dynamic repertoire that could mediate DNA–protein assemblies. R_1ρ relaxation dispersion NMR methods are powerful tools to capture such slow conformational exchanges in solution using ¹³C/¹⁵ N labelled DNA. Here, these approaches were applied to a dodecamer containing a TTAAA element that was assumed to facilitate nucleosome formation. NMR data and inferred exchange parameters assign HG base pairs as the minor, transient conformers specifically observed in three successive A·T base pairs forming the TAA·TTA segment. The abundance of these HG A·T base pairs can be up to 1.2% which is high compared to what has previously been observed. Data analyses support a scenario in which the three adenines undergo non-simultaneous motions despite their spatial proximity, thus optimising the probability of having one HG base pair in the TAA·TTA segment. Finally, revisiting previous NMR data on H2 resonance linewidths on the basis of our results promotes the idea of there being a special propensity of A·T base pairs in TAA·TTA tracts to adopt HG pairing. In summary, this study provides an example of a DNA functional element submitted to slow conformational exchange. More generally, it strengthens the importance of the role of the DNA sequence in modulating its dynamics, over a nano- to milli-second time scale.

The nucleocapsid domain (NCd), located at the C-terminus of the HIV-1 Gag protein, is involved in numerous stages of the replication cycle, such as the packaging of the viral genome and reverse transcription. It exists under different forms through the viral life cycle, depending on the processing of Gag by the HIV-1 protease. NCd is constituted of two adjacent zinc knuckles (ZK1 and ZK2), separated by a flexible linker and flanked by disordered regions. Here, conformational equilibria between a major and two minor states were highlighted exclusively in ZK2, by using CPMG and CEST NMR experiments. These minor states appear to be temperature dependent, and their populations are highest at physiological temperature. These minor states are present both in NCp7, the mature form of NCd, and in NCp9 and NCp15, the precursor forms of NCd, with increased populations. The role of these minor states in the targeting of NCd by drugs and its binding properties is discussed.