Polyproline is a chain or sequence region enriched in repeated proline residues. Proline constrains backbone geometry, and some proline-rich regions adopt an extended polyproline-II conformation. Such regions can serve as recognition elements for interaction domains in signalling proteins. Sequence enrichment, conformation and binding function should be distinguished: a proline-rich segment does not automatically establish one fixed structure or partner. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 3, Protein Structure: backbone conformation and protein-interaction domains
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