Evolution, disordered proteins

The next level up in protein organization involves secondary structures motifs—small sfrucfural elements that show up repeatedly within many different protein folds. Well-known examples include helices, sfrands, loops, and furns. Work in yeasf has shown thaf fhe secondary strucfure composifion of a profein does nof appear fo influence ifs evolutionary rafe [7]. However, in a sfudy of mammalian profeins, residues in helices and sfrands were shown fo evolve more slowly fhan those in the less ordered loops and turns [53]. This last result highlights another apparent influence on evolution molecular disorder. Disordered regions of proteins are generally known to evolve more rapidly than their ordered counterparts [61]. Our discussion to this point has assumed that a useful sfrucfure and a stable fold are synonymous—this is not necessarily the case. In fact, many proteins perform functional roles in the cell despite the fact that they, either in whole or in part, fail to achieve a fixed three-dimensional fold [62]. The facf fhaf fhese profeins also appear fo experience relaxed selection raises inferesfing quesfions abouf fheir evolutionary pofenfial. 

Disordered (unfolded) regions of a protein are known to perform important biological functions, in spite of relaxed constraint on their three dimensional structures. It has been shown that hub proteins, which are believed to be constrained by coevolution with their interaction partners, are also more likely to feature intrinsic disorder [87]. This provides another example of a pair of deterministic forces in evolution with a paradoxical relationship. A recent work by Kim et al. addresses this issue by considering the precise physical context of disordered regions that occur in interacting proteins [88]. 

All these chiral structural standards and the corresponding families around them in proteins and nucleic acids represent, however, only the main building blocks upon which evolution has been operating. The colored loop- and knot-stretches and all the more disordered parts of protein and nucleic acid structural designs are of primary importance for the evolutionary aspects of informational and functional processings in our evolving life patterns. 

Schlessinger, A., Schaefer, C., Vicedo, E., et al. (2011) Protein disorder—a breakthrough invention of evolution Curr Opin Struct Biol, 21 (3), 412-418. 

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