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Viral capsids are icosahedral or helical protein shells composed of many copies of one or a few protein types. The capsid protects the viral genome and determines virion shape and stability. Assembly is often spontaneous in vitro for simple viruses but assisted by scaffolding proteins and enzymatic maturation processes in cells.
From your study of the viral replication cycle, you know that a virus must package its genome into a protective particle before leaving the host cell. The structure responsible for this protection is the capsid — a protein shell built from many copies of one or a few protein subunits called capsomeres. The capsid's design solves a fundamental engineering problem: how to enclose a large nucleic acid molecule using the smallest possible amount of genetic information.
Two basic architectural solutions have evolved. Icosahedral capsids use 20 triangular faces arranged into a roughly spherical shape, the same geometry seen in a soccer ball. This design is extremely efficient because identical protein subunits can be arranged symmetrically to create a closed shell, with the number of subunits following precise mathematical rules described by the triangulation number (T-number). A T=1 capsid uses 60 subunits; larger capsids like adenovirus use T=25, requiring 1,500 copies arranged in slightly different local environments. The second solution is the helical capsid, where protein subunits spiral around the nucleic acid like steps in a staircase. Tobacco mosaic virus is the classic example — its rod-shaped particle is simply a helix of identical coat proteins wound around the RNA genome.
Your knowledge of protein quaternary structure helps explain why capsid assembly can be remarkably self-directed. The subunit interfaces are encoded in the protein's shape — complementary surfaces, hydrophobic patches, and electrostatic interactions guide each subunit into its correct position. For simple viruses like TMV, purified coat protein and RNA will spontaneously assemble into infectious particles in a test tube, demonstrating that all the assembly information is contained in the components themselves. More complex viruses, however, require scaffolding proteins that act as temporary templates during assembly and are removed or degraded in the final particle. Many bacteriophages and herpesviruses use this strategy.
After initial assembly, many capsids undergo a maturation step that dramatically changes their properties. In HIV, for example, the immature capsid is a spherical lattice of Gag polyproteins. After budding, the viral protease cleaves Gag into its component domains, which rearrange into the characteristic conical mature capsid. This maturation is essential for infectivity — protease inhibitor drugs exploit this dependency by blocking the cleavage step, producing non-infectious particles. The capsid is therefore not just passive packaging; it is a dynamic molecular machine whose structure determines viral stability in the environment, receptor interactions during entry, and the timing of genome release inside new host cells.