The symmetry-mismatched viral vertex, therefore, is central to driving key assembly steps that build a virus particle of precise dimensions, as well as its infectivity.
When infection occurs, the portal vertex acts again, releasing the viral genome into a new host cell through the same channel that is now aligned with a tail tube 10, 11. Sensing the fullness of genome-packaged head, the portal vertex then expels the packaging motor and binds to head-tail connector proteins that seal off the head and attach an infection machine such as the tail. Next, by interacting with a motor protein, it assembles a powerful packaging machine that translocates viral genome through its ~35 Å central channel 4, 6. It is also the first-assembled virus structural unit, which nucleates capsid (head) assembly 9. It is an extreme form of symmetry mismatch where the sub-assemblies, the portal and the capsid, have completely different structures, symmetries, and functions. The viral portal vertex consists of a 12-fold-symmetric dodecameric portal complex inserted into a 5-fold-symmetric capsid vertex, making it the unique vertex of an icosahedral viral capsid 2, 8. Phylogenetic analyses indicate that it is one of the most ancient structures, dating back to an early time in evolution when self-replicating entities appeared on Earth 7. It is universally found in tailed bacteriophages (phages), herpesviruses, and archaeal viruses, and is structurally well-conserved 4, 5, 6.
Of these, the viral portal vertex represents an extreme form, and probably the most widely distributed symmetry-mismatched structures in the biosphere 2, 3. Examples include viruses and numerous cellular complexes including, cytoskeletons, secretion systems, and so on. It introduces flexibility and dynamism that are fundamentally important for executing biological functions. Symmetry mismatches in macromolecular assemblies are ubiquitous in biological systems 1.
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