Chemically modified tandem repeats in proteins: natural combinatorial peptide libraries.

Many proteins composed of tandem repeats (a linear motif, directly repeated within the sequence) are substrates for post-translational modifications (PTMs). Tandem repeats are also dynamic in number, presumably due to instability in the underlying DNA sequence. These observations lead to a hypothesis that cells use a combination of PTMs and variability in repeat number to mediate protein function. Evidence of these processes co-regulating diverse aspects of cellular function can be found in all organisms from bacteria to humans, suggesting a common but poorly described mechanism for regulating and diversifying protein function. This review highlights several examples whereby protein modifications and repetitive protein domains impart diversity. Lastly, it speculates on the possibility of using chemically modified repetitive amino acid sequences to develop peptide-based biomolecules with novel functions.