PQBP1 couples HIV-1 capsid recognition to cGAS recruitment through conformational remodeling

Technical Summary

Problem Statement
Pattern recognition receptors (PRRs) are essential for selectively engaging pathogen-derived signals to initiate inflammation and antimicrobial responses. In the context of HIV-1 infection, Polyglutamine-binding protein 1 (PQBP1) acts upstream of cyclic GMP-AMP synthase (cGAS) by recognizing the viral capsid. However, the specific molecular mechanism by which PQBP1's binding to the capsid facilitates the subsequent recruitment of cGAS has remained unclear. This gap is particularly significant given that PQBP1 is an intrinsically disordered protein (IDP), which typically exists as an ensemble of conformational states rather than a single rigid structure, complicating the understanding of how ligand binding translates into functional output.

Methodology
The study investigates the conformational dynamics of PQBP1 upon interaction with the HIV-1 capsid. The authors analyze how capsid engagement alters the structural ensemble of PQBP1, specifically focusing on the interplay between the N-terminal pore-binding region, the WW domain, and the polar-rich domain (PRD). To validate the functional necessity of these conformational changes, the researchers employed site-directed mutagenesis, creating alanine substitutions in regions identified as responsive to capsid binding. These mutants were then tested for their ability to bind the capsid versus their capacity to recruit cGAS during infection.

Key Results
The study demonstrates that capsid engagement does not merely lock PQBP1 into a static state but rather shifts the population of conformational states toward a specific conformation competent for cGAS binding. Key findings include:

• Conformational Redistribution: Binding of the capsid to the N-terminal pore-binding region induces a redistribution of conformational sampling within the WW domain.
• Long-Range Effects: This binding event triggers long-range conformational changes, evidenced by the shielding of distal regions within the polar-rich domain (PRD) from the solvent.
• Functional Separation: Alanine substitutions in these capsid-responsive regions retained the ability to bind the capsid but failed to support infection-dependent cGAS recruitment. This confirms that capsid binding and cGAS recruitment are separable functions of PQBP1, dependent on specific conformational states rather than just the physical presence of the protein.

Significance and Claims
The paper defines a mechanism wherein N-terminal capsid engagement is transduced through the WW domain to the PRD, effectively positioning PQBP1 for cGAS recruitment during the innate sensing of the HIV-1 capsid. The authors claim that this work elucidates how an intrinsically disordered protein utilizes conformational remodeling to couple pathogen recognition with downstream immune signaling. By establishing that the transition from capsid binding to cGAS recruitment is a conformationally gated process, the study provides a structural and mechanistic basis for understanding how PQBP1 functions as a critical bridge in the innate immune response to HIV-1.