Understanding how viruses build their protective protein shells—and how that construction can be disrupted—could support the development of new antiviral drugs, improved vaccines and other treatments. The discovery helps explain how individual proteins consistently organize into the correct structure despite the thousands of ways they could potentially connect.

The researchers found that virus assembly resembles a complex maze in which only certain paths lead to lasting progress. At first, protein components form weak bonds and can easily separate if they are misaligned. This flexibility allows the molecules to try different combinations until they form a specific closed structure. Multiple connections then hold these structures together and make them far more stable.

These stable configurations act as molecular checkpoints, reducing the vast number of possible assembly pathways to a much smaller set of successful routes. One especially important milestone occurs when five larger protein components join to form a closed pentagonal ring. This is the first highly stable structure in the assembly process. Once it appears, each subsequent stabilization stage requires fewer additional components, allowing the particle to grow more quickly.

Co-first author Dr Roy Athol, from the University of Oxford’s Department of Chemistry and the Kavli Institute for Nanoscience Discovery, explained:

“Viruses have to solve extraordinary structural problems; their components must somehow find the right arrangement among a huge number of possibilities, without a blueprint or machinery to direct the process. We can now observe what happens molecule by molecule and see the physical laws that make this possible.”

How virus-like particles assemble molecule by molecule

Viruses store and protect their genetic material inside an organized protein shell known as a capsid. Depending on the virus, a capsid may contain tens or even thousands of individual protein components. Remarkably, these proteins can often arrange themselves into the correct structure without external machinery directing the process.

Scientists have studied viral self-assembly for decades, but observing the process as it happens has been a major challenge. Many intermediate structures exist only briefly and occur in very small amounts. Traditional techniques have therefore generally shown researchers the starting materials and finished particles without clearly revealing how one changes into the other.

To overcome this limitation, the Oxford team studied an artificial virus-like particle made from 60 protein units. The researchers used mass photometry, a technique developed at Oxford that determines the mass of individual molecules by measuring the light they scatter.

They combined mass photometry with a new method that confines individual molecules and allows them to be observed continuously. This enabled the team to repeatedly measure particle mass as additional proteins attached during assembly.

The approach allowed the researchers to follow construction in situ, effectively tracking the addition of individual molecular building blocks over time.

Co-lead author Dan Lowenthal, a PhD student at the University of Oxford’s Department of Chemistry and the Kavli Institute for Nanoscience Discovery, said:

“This method allows us to study the assembly process directly—just take a video!”

Co-lead author Professor Philip Kukla, from the University of Oxford’s Department of Chemistry and the Kavli Institute for Nanoscience Discovery, said:

“Until now, much of our understanding of how these structures are assembled has had to be reconstructed from snapshots or theoretical models. Things change when we can quantify the underlying interactions and track single particles as they grow. We can see important intermediate structures emerge and can extrapolate detailed models of the assembly process.”

Why successful virus assembly depends on trial and error

The findings help explain how a process involving an enormous number of possible molecular arrangements can consistently produce the correct structure. Instead of exploring every possible route, the proteins move through a relatively small number of stable intermediate stages.

The results also show why the early stage of assembly, known as nucleation, is particularly difficult—and why growth accelerates after the first stable structure forms.

A key part of this efficiency is the ability of protein components to make mistakes without permanently halting the process. Weak initial binding causes incorrect combinations to separate, giving the proteins another opportunity to find a better configuration. Once enough components connect in the correct closed arrangement, the structure becomes stable enough to support further growth.

Dr Athol added:

“Weak interactions give the system room for error. Most encounters do not need to be successful; components can be separated and tried again. However, when a sufficient number of components join together in the proper closed arrangement, the structure stabilizes and assembly can move forward. The combination of trial and error and subsequent fixation of a successful structure makes the process reliable.”

Potential applications for antiviral drugs and vaccines

By identifying the molecular interactions that control virus assembly, scientists may eventually be able to develop treatments that interfere with the construction of viral protein shells. The same insights could support efforts to engineer virus-like particles for vaccines and other therapeutic applications.

Co-author Dr Jack Tan, from the MRC Weatherall Institute of Molecular Medicine and the CAMS Oxford Institute, said:

“The ability to understand these processes at the molecular level could have important applications in vaccine development and antiviral drugs that disrupt viral assembly.”

The implications could extend beyond viruses. Self-assembly is essential throughout biology and helps create structures ranging from the internal skeleton of cells to specialized molecular compartments and large protein complexes.

By making it possible to observe complex assembly processes one molecule at a time, the University of Oxford team’s approach provides a powerful new way to study how intricate biological structures arise from relatively simple molecular interactions.