Assembly of exons from unitary transposable genetic elements: implications for the evolution of protein-protein interactions.

The discovery of "genes-in pieces" provided the first evidence that modern proteins evolved through the assembly and shuffling of simpler building blocks-generally equated with exons. In the theoretical model presented here, it is suggested that exons were created from even smaller modules that have been termed duplication units. Furthermore, these segments may represent the ultimate building blocks for protein assembly. The nucleotide sequences of the duplication units to appear to resemble those mobile genetic elements such as transposons or insertion sequences, i.e. they possess direct repeats at each end and inverted sequences extending 15-25 base pairs from these direct repeats. During evolution, these transposable exons (trexons) would have been replicated and dispersed in the genome thereby promoting homologous recombination and further duplication. Thus, the transposition and splicing of these gene segments gave rise to increasingly complex proteins as well as multi-gene families of proteins. It has been proposed that peptides encoded by the first trexons were predisposed to form dimers or oligomers. Detailed structural analysis of various protein-protein complexes has revealed a tendency for the duplication units to self-associate. Self-binding peptides could have ultimately led to the evolution of protein ligands and receptors with high affinity.