The Mechanism of Conformational Transition and Stabilisation Strategies of Viral Membrane Fusion Proteins

Viral membrane fusion proteins facilitate the fusion of viral and host cell membranes by undergoing a transition from a prefusion conformation to a post-fusion conformation, thereby enabling the transfer of viral nucleic acids into the cell interior. This transition process is characterized by peptide exposure, membrane insertion, and structural refolding. The prefusion configuration represents an optimal target for vaccine formulation and antiviral pharmacotherapy. However, the metastable nature of the prefusion conformation makes it prone to spontaneous conversion into the stable post-fusion conformation, thereby complicating structural analysis and vaccine design. Investigating the mechanisms of conformational change in these proteins and developing methods to stabilize their prefusion state remain challenging research topics. This review summarizes the structural and functional differences among three classes of membrane fusion proteins: class I proteins, which are predominantly composed of α-helices, form trimers, and rely on receptor binding or low pH to trigger fusion peptide release; class II proteins, which are mainly β-sheet-rich, rearrange from dimers to trimers and activate fusion loops via low pH; and class III proteins, which combine α-helices and β‑structures, with mechanisms involving internal fusion loop insertion and membrane remodeling. It is evident that a comprehensive understanding of the mechanisms underlying viral membrane fusion is crucial for developing effective stabilization strategies for the prefusion conformation of these proteins. This paper presents several such methods that have been successfully employed in this endeavor, including: disulfide bond formation to stabilize domain-domain interactions; hydrophobic cavity filling to enhance core stability; proline substitution to restrict structural transitions in hinge regions; and multimer domains stabilizing the trimeric conformation. The stabilization strategies summarized and discussed herein have been validated in studies of multiple viral membrane fusion proteins and further applied in the design of vaccine antigens. Moreover, this paper highlights the potential applications of novel techniques, such as time-resolved cryo-EM, in capturing conformational intermediates and resolving dynamic transition processes. Such stabilization efforts, informed by structural insights, have yielded promising outcomes—for instance, prefusion-stabilized RSV F antigens that elicit potent neutralizing antibodies in clinical trials. Looking ahead, integrating computational modeling, such as AlphaFold predictions, with experimental data will further refine these approaches. Ultimately, these innovations promise to enable structure-guided therapeutics to combat emerging viral threats. This review provides a theoretical foundation for developing stable viral membrane fusion proteins, offering crucial insights for understanding viral membrane fusion mechanisms and advancing next-generation vaccines and antiviral drugs.