Viral disorder or disordered viruses: do viral proteins possess unique features?

Many proteins or their regions are disordered in their native, biologically active states. Bioinformatics has revealed that these proteins/regions are highly abundant in different proteomes and carry out mostly regulatory functions related to molecular recognition, signal transduction, protein-protein, and protein-nucleic acid interactions. Viruses, these "organisms at the edge of life", have uniquely evolved to be highly adaptive for fast change in their biological and physical environment. To sustain these fast environmental changes, viral proteins elaborated multiple measures, from relatively low van der Waals contact densities, to inclusion of a large fraction of residues that are not arranged in well-defined secondary structural elements, to heavy use of short disordered regions, and to high resistance to mutations. On the other hand, viral proteins are rich in intrinsic disorder. Some of the intrinsically disordered regions are heavily used in the functioning of viral proteins. Others likely have evolved to help viruses accommodate to their hostile habitats. Still others evolved to help viruses in managing their economic usage of genetic material via alternative splicing, overlapping genes, and anti-sense transcription. In this review, we focus on structural peculiarities of viral proteins and on the role of intrinsic disorder in their functions.