Harvard scientists have settled a long-standing debate about how viruses assemble, according to a paper published in the Proceedings of the National Academy of Sciences on Sept. 30.
Chemical Engineering and Physics Professor Vinothan N. Manoharan led the small team — comprising Applied Physics research associate Rees F. Garmann and former Applied Physics graduate student Aaron M. Goldfain — working on the project.
The specific virus they examined consists of an RNA strand of about 3600 nucleotides, bounded by a protective capsule — known as a capsid — of 180 proteins, according to Manoharan.
Manoharan and Garmann said the team sought to settle a debate between two theories of viral assembly. The first theory — called the “en masse” pathway — posits that most of the requisite proteins glom onto to the RNA in a disordered way, and then subsequently sort themselves into a capsid. The second theory — termed the nucleation and growth model — suggests that a few proteins drift onto and off from the RNA molecule, at some point forming a critical mass — or nucleus — from which the capsid proceeds to grow rapidly.
Manoharan added that past experimentation has not been able to resolve the question.
“People had done experiments looking at many viruses assembling at the same time, and kind of averaging over all of that data. But that doesn't give you enough information to see which of these pathways actually is actually working,” he said.
Manoharan said that his team harnessed the ultra-sensitive technique of interferic scattering microscopy in order to see the tiny individual viruses — which span only 30 nanometers in diameter.
What the team saw, he added, supports the nucleation and growth model for small RNA viruses.
“It could take minutes before the nucleus forms — the nucleus is the cluster of proteins on the RNA. But then once that nucleus forms, the virus grows in a matter of seconds,” he said.
Manoharan added that the team made one further observation — namely, that a high protein concentration can induce multiple protein nuclei to form on the RNA, creating multiple shells that fail to merge together.
“If you get more than one nucleus forming on the RNA before the assembly is complete, you end up with these monster structures, which are imperfect and wouldn’t serve the virus very well,” Garmann said.
Manoharan said this observation has a potential clinical upshot.
“It suggests that one way to disrupt viral assembly might actually be to increase the rate at which these nuclei are forming, which is a bit counterintuitive, because it would correspond to increasing the amount of viral protein that's inside your system,” he said.
Garmann said he spots new questions on the horizon.
“How does the RNA affect the nucleation process? How does it either promote or facilitate it?” he asked.