How does the SV40 capsid achieve T = 7 quasi-symmetry without the expected number of identical subunits?

Sourav PanOctober 28, 2024

Answered

The SV40 capsid achieves T = 7 quasi-symmetry through a unique arrangement of its protein subunits, specifically the VP1 proteins, which allows it to maintain the structural integrity and symmetry of the capsid without the expected number of identical subunits. Here are the key points explaining how this is accomplished:

1. Pentameric Arrangement: The SV40 capsid is composed of 72 pentamers of the VP1 protein. Each pentamer consists of five VP1 subunits, which are arranged in a way that allows for the formation of a quasi-symmetrical structure. This arrangement is crucial for achieving the T = 7 symmetry, which is characterized by a specific geometric configuration of pentamers and hexamers.
2. Non-Identical Subunit Environments: In a true T = 7 lattice, one would expect a specific arrangement of pentamers and hexamers. However, in SV40, 60 of the pentamers are located in positions that would typically be occupied by hexamers in an ideal T = 7 lattice. This means that the capsid can achieve the necessary symmetry without having the full complement of identical subunits that would normally be required.
3. Covalent and Non-Covalent Interactions: The interactions between the VP1 subunits, including both covalent (e.g., disulfide bonds) and non-covalent interactions (e.g., hydrogen bonds, hydrophobic interactions), help to stabilize the pentameric structures and their arrangement. These interactions allow the pentamers to effectively substitute for hexamers in certain positions, maintaining the overall symmetry of the capsid.
4. Flexibility of C-Terminal Arms: The C-terminal arms of the VP1 proteins provide flexibility and adaptability during the assembly process. This flexibility allows the arms to reach out and form interactions with neighboring pentamers, facilitating the assembly of the capsid in a way that achieves the desired quasi-symmetry.
5. Quasi-Equivalence Theory: The assembly of the SV40 capsid can be understood through the principles of quasi-equivalence, which suggest that viral capsids can be constructed from fewer types of subunits than would be expected based on strict symmetry. This allows for variations in the arrangement of subunits while still achieving a stable and functional structure.
6. Unique Structural Features: The specific structural features of the VP1 proteins, including their folding patterns and the arrangement of their secondary structures, contribute to the ability of the capsid to achieve T = 7 quasi-symmetry. The unique folding of the VP1 proteins allows them to interact in a way that supports the overall architecture of the capsid.