Omnipresence of the polyproline II helix in fibrous and globular proteins

[Display omitted] •The polyproline II helix is omnipresent.•Role of proline in stability of PPII helix and its interactions.•Specific mechanisms work in PPII structural organization and thermostability.•Diverse structural assemblies of the PPII and their functions.•PPII-based drug and artificial collagens are indispensable in biomedicine. Left-handed helical conformation of a polypeptide chain (PPII) is the third type of the protein backbone structure. This conformation universally exists in fibrous, globular proteins, and biologically active peptides. It has unique physical and chemical properties determining a wide range of biological functions, from the protein folding to the tissue differentiation. New examples of the structure have been appearing in spite of difficulties in their detection and investigation. The annotation and prediction of the PPII was also a challenging task. Recently, many PPII motifs with new and/or unexpected functions are being accumulated in databases. In this review we describe the major structural and dynamic forms of PPII, the diversity of its functions, and the role in different biological processes.