Physical model describes structures of viral capsids

The main conclusion of the study is that viral capsids can only adopt a finite range of radii, lengths and protein numbers, making it possible to calculate and characterize all of their possible structures. "This model marks an important step towards understanding the viral assembly process and opens the way for controlling this process for applications in biotechnology, such as gene therapy, and applications in nanotechnology, for example in the creation of nanoscale moulds with highly precise dimensions for designing nanostructures," explains David Reguera.

Viral capsids are formed through a process of self-assembly governed by a universal physical principle: energy minimization. Based on this knowledge, it was possible to identify the potentially optimal architectures of viral capsids; that is, those structures which minimize the energy requirement. As Reguera explains, "we have found that the well-defined geometry observed in different spherical and bacilliform viruses is a product of free-energy minimization in the interaction between the different structural units of which the capsid is composed."

Since the 1960s scientsts have known that spherical viruses adopt a clearly defined structure with icosahedral symmetry, formed by groups of six and five proteins (hexamers and pentaments, respectively), similar to the panel structure of a football, for example. In the case of bacilliform viruses, however, the structure had not been clearly identified. The results of this new study suggest that the capsids of bacilliform viruses are generally formed by a tube-like central body, the ends of which are closed by isocahedral caps centred on one of the three axes of symmetry. These structures are similar to those of fullerenes and carbon nanotubes and have the advantage of being highly stable and resistant.

Reguera and Luque, with support from the researcher Roya Zandi, of the University of California, applied a simple physical model and found that the local energy is minimal for bacilliform capsids formed by a specific, discrete number of proteins distributed in a cylindrical body of hexamers and closed by isocahedral caps centred along the 5-, 3- and 2-fold axes.

The study corroborates the existence of this type of viral structure and, with the complimentary geometric model, serves as the basis for reproducing the architecture of spherical and bacilliform viruses in vivo and in vitro and for making informed predictions. The models have been successfully applied to several known viruses and confirm many of the hypotheses from earlier studies regarding the structure of the alfala mosaic virus, which adopts different lengths depending on the quantity of genetic material contained. Given that the different lengths correspond to the rules set out in the model, it has been possible to obtain definitive models of the finite possible structures.