⚛️ biophysics

Psi RNA-specific Binding Promotes HIV-1 Gag Conformational Change Critical for Immature Viral Particle Assembly

This study demonstrates that the specific binding of HIV-1 Gag to the Psi RNA packaging signal, in conjunction with the host co-factor IP6, drives a critical conformational shift from a compact to an extended state essential for immature viral particle assembly.

Original authors: Qiu, Y., Banerjee, P., Grabarkewitz, K., Wysocki, V. H., Rouzina, I., Voth, G. A., Musier-Forsyth, K.

Published 2026-07-20

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CC BY 4.0

Original authors: Qiu, Y., Banerjee, P., Grabarkewitz, K., Wysocki, V. H., Rouzina, I., Voth, G. A., Musier-Forsyth, K.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine the human body as a bustling city, and inside that city, a tiny, sneaky intruder called HIV-1 is trying to build a secret fortress. To do this, the virus needs to assemble thousands of tiny building blocks, called proteins, into a perfect shell. But here's the tricky part: these building blocks are like shape-shifting robots. Sometimes they curl up into a tight, compact ball (the "C" state), and sometimes they stretch out into a long, lanky pose (the "E" state). For the virus to build its fortress correctly, these robots must stretch out. If they stay curled up, the fortress collapses, and the virus can't infect anyone.

Scientists have known for a while that the virus uses a special "instruction manual" made of RNA to tell these robots what to do. They also knew that a common molecule found in our cells, called IP6, acts like a helpful assistant to speed things up. But there was a big mystery: How exactly does the virus know which RNA to listen to? The city is full of millions of different RNA messages, yet the virus only grabs the specific one it needs. Does the RNA just stick to the robot, or does it actually force the robot to change its shape? And does this shape-shifting happen before the robots start building, or only after?

This paper dives right into that mystery. The researchers, led by a team at Ohio State University and the University of Chicago, wanted to see if the specific viral RNA (called Psi RNA) acts like a magic key that unlocks the robot's ability to stretch out. They didn't just guess; they built a special version of the HIV-1 building block (the Gag protein) with two tiny glowing lights attached to its ends—one at the head and one at the tail. When the robot curls up, the lights are close together and glow brightly in a specific way (high FRET). When it stretches out, the lights move apart, and the glow changes (low FRET).

By watching these glowing robots in a test tube, the team discovered that without any help, the robots mostly stay curled up in their compact "C" state. However, when they added the specific Psi RNA, the robots suddenly stretched out much more than they did when exposed to other, non-viral RNAs. It's as if the Psi RNA didn't just sit on the robot; it gave it a gentle nudge that forced it to stand up tall and ready for work. The study also found that the helper molecule, IP6, could make the robots stretch out on its own, even without RNA, but the specific RNA made the effect even stronger.

To make sure this wasn't just a fluke, the team ran super-complex computer simulations. These digital models showed exactly how the change happened: the RNA grabbed onto a specific part of the robot (the NC domain), which caused the robot's internal "hinges" to loosen up. This allowed the head and tail to unbind from each other, letting the whole thing snap into that long, extended shape needed for assembly. The simulations confirmed that this stretching is driven by the flexibility of the robot's middle section and the release of its head from its tail.

The researchers also tested how well these robots stick together to form pairs (dimers). They found that when the robots were holding the specific Psi RNA, they were much more likely to pair up and start building the virus shell compared to when they held other RNAs. In fact, the specific RNA made the robots about 1.4 times more likely to be in the "ready-to-build" extended state than other RNAs, and when combined with the helper molecule IP6, the difference in how well they stuck together was huge—about 36 times more likely to form pairs on the specific RNA.

So, what's the takeaway? The virus doesn't just randomly grab RNA; it uses the specific viral RNA as a trigger. This RNA binding acts like a switch that flips the building blocks from a safe, compact ball into an assembly-ready, stretched-out shape. This shape-shifting is critical because it helps the virus pick the right genetic instructions and start building its fortress at the right time. While the study suggests this mechanism is a key part of how HIV-1 works, it also hints that the compact "C" state might have other jobs in the virus's life cycle that we haven't figured out yet. By understanding exactly how these shape-shifters work, scientists hope to find new ways to jam the gears of the virus, stopping it from building its fortress before it can even start.

Technical Summary: Psi RNA-specific Binding Promotes HIV-1 Gag Conformational Change Critical for Immature Viral Particle Assembly

Problem Statement
The assembly of the immature HIV-1 virion relies on the Gag polyprotein, which must selectively package the dimeric genomic RNA (gRNA) containing the Psi ( $\Psi$ ) packaging signal amidst a vast excess of cellular RNAs. While the nucleocapsid (NC) domain of Gag binds the Psi signal, in vitro binding affinities for Psi and non-Psi RNAs are often similar under physiological conditions. Furthermore, Gag exists in solution in two primary conformational states: a compact (C) state and an extended (E) state. Previous research established that only the E-state Gag forms virus-like particles (VLPs) of the correct size, and that the host cell factor inositol hexakisphosphate (IP6) shifts the equilibrium toward the E-state. However, the specific influence of RNA binding—particularly the distinction between Psi and non-Psi RNAs—on the C-to-E conformational transition of full-length monomeric Gag remained unknown.

Methodology
The study employed an integrated approach combining ensemble Förster resonance energy transfer (FRET), mass photometry (MP), and all-atom molecular dynamics (AAMD) simulations.

Protein Engineering and Labeling: A full-length wild-type (WT) Gag and a monomeric WM-Gag variant (W316A/M317A mutations) were engineered with an N-terminal triglycine motif and a C-terminal KCK motif. These were dual-labeled with Cy3 (donor) at the N-terminus and Cy5 (acceptor) near the C-terminus to enable FRET measurements of the end-to-end distance.
FRET Assays: Ensemble FRET efficiency was measured to monitor Gag conformation. Experiments were conducted under varying conditions:
- Salt titration (100–500 mM NaCl) to probe electrostatic stabilization of the C-state.
- Titration with specific RNAs: Psi RNA (genomic packaging signal), TARpA (HIV-1 5'UTR element), and tRNA $^{Lys3}$ (primer).
- Titration with the assembly co-factor IP6, both alone and in combination with RNAs.
- Competitive binding assays where Gag was pre-incubated with tRNA before titration with Psi or TARpA.
- Concentration-dependent studies to assess dimerization.
Mass Photometry (MP): MP was used to directly visualize and quantify the stoichiometry of Gag-RNA complexes, distinguishing between monomeric and oligomeric species bound to Psi versus TARpA.
Molecular Dynamics Simulations: Long-timescale (1 $\mu$ s) all-atom MD simulations were performed for monomeric Gag in solution, both in the absence of RNA and in the presence of Psi RNA bound to the NC domain. These simulations tracked end-to-end distances and inter-domain interactions (specifically MA-CA and NC-CA contacts).

Key Results

Conformational States and Electrostatics: In low salt, dual-labeled WT Gag exhibited high FRET efficiency (0.55), indicating a predominantly compact (C) state. Increasing salt concentration reduced FRET, shifting the equilibrium toward the extended (E) state, suggesting the C-state is stabilized by weak electrostatic intra-Gag interactions. The WM-Gag mutant showed significantly lower FRET (0.22) even in low salt, indicating the WM site is critical for stabilizing the C-state via intramolecular contacts.
RNA-Induced Conformational Shifts: Binding of all tested RNAs induced a decrease in FRET (shift toward E-state). However, Psi RNA induced the most significant shift, reducing the fraction of C-Gag to ~0.44, compared to ~0.54 for tRNA and ~0.65 for TARpA. This corresponds to a lower free energy barrier for the C-to-E transition when Gag is bound to Psi.
Role of IP6: IP6 alone promoted the E-state, reducing the C-Gag fraction from ~~0.95 to ~0.74. In the presence of IP6, the dissociation constants ( $K_d$ ) for RNA binding became measurable, revealing that Gag binds Psi RNA (~~36 nM) approximately 4-fold more tightly than TARpA (~162 nM).
Competition and Oligomerization: Pre-incubation with tRNA (mimicking cellular conditions) did not prevent Psi from inducing the E-state shift but increased the binding affinity difference between Psi and TARpA to ~6-fold. Mass photometry demonstrated that Psi RNA facilitated the formation of higher-order Gag oligomers (up to 5 Gag molecules per RNA dimer), whereas TARpA primarily formed 1:1 complexes.
Simulation Insights: AAMD simulations confirmed that free Gag monomers spontaneously collapse into a compact state. In the presence of Psi RNA, the simulations captured spontaneous transitions to the extended state. The mechanism involves the NC domain binding Psi RNA, which disrupts stabilizing interactions between the NC domain and the MA/CA domains. Specifically, the transition is driven by the orientational flexibility of the CA linker and the unbinding of the MA domain from the CA N-terminal domain (CANTD).

Significance and Claims
The authors claim that this work provides the first direct experimental evidence that specific binding of Psi RNA to the NC domain of Gag promotes a conformational shift from the compact (C) to the extended (E) state, a prerequisite for viral assembly. The study establishes that:

The C-state is stabilized by a combination of electrostatic interactions and hydrophobic contacts involving the WM site in the CA C-terminal domain.
Specific Psi RNA binding, in conjunction with host factors like IP6 and tRNA, leverages the flexibility of the multi-domain Gag polyprotein to stabilize the assembly-competent E-state.
This conformational switch is a critical step in the selective packaging of the viral genome, as Psi-bound Gag is not only more likely to be in the extended state but also more prone to oligomerization.
The integrated experimental and computational approach elucidates a molecular mechanism where viral (Psi RNA) and host (IP6, tRNA) factors cooperate to orchestrate the progression of HIV-1 assembly.

The paper concludes that while specific RNA binding enhances dimerization propensity, the magnitude of this effect (~72-fold enhancement relative to non-specific RNA in theoretical estimates) suggests that other factors likely contribute to the high fidelity of gRNA packaging in the cellular environment.

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