A central dogma of molecular biology is that the sequence of a protein dictates its particular fold and the fold dictates its function. Indeed, the sequence → structure → function hypothesis has been a guiding principle by which scientists approach molecular biology. Every student knows that the genome encodes information for the progression from primary sequence to secondary, tertiary, and ultimately quaternary structure. Yet with a growing number of proteins, a fifth level has been identified: rearrangement of existing structures into distinct forms. Recent observations indicate that replication of Ebola virus depends on this fifth level. We believe other viruses with compact genomes and rapid evolution under selective pressure will be a rich source of examples of polypeptides that rearrange to gain added functions. In this review, we describe mechanisms by which viral, prokaryotic, and eukaryotic polypeptides have adopted alternate structures to control or gain function.

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