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Function driven protein evolution. A possible proto-protein for the RNA-binding proteins.

We introduce a hypothesis that present day proteins evolved from "proto-proteins," small 15-20 residue peptides with some elements of secondary structure and primitive function. Increasingly stable and functional proteins arose by adding structural elements to produce the small domains or protein modules that we would recognize today. From this point of view, the surprising similarities between small structural fragments of large proteins, that are usually taken as examples of convergent, function-driven evolution, are interpreted in exactly the opposite way--as traces of common evolutionary origin. As an example, a hypothetical evolutionary tree for two families of RNA binding proteins, the OB fold, a family of all beta proteins, and RBD fold, an alpha/beta protein family is presented. We argue that both protein families could have evolved from the same RNA-binding proto-protein, which had a form of beta-loop-beta RNA binding motif.