Polyproline II Helix as a Recognition Motif of Plant Peptide Hormones and Flagellin Peptide flg22.

Plant peptide hormones play a crucial role in plant growth and development. A group of these peptide hormones are signaling peptides with 5 ⎼ 23 amino acids. Flagellin peptide (flg22) also elicits an immune response in plants. The functions are expressed through recognition of the peptide hormones and flg22. This recognition relies on membrane localized receptor kinases with extracellular leucine rich repeats (LRR⎼RKs). The structures of plant peptide hormones ⎼ AtPep1, IDA, IDL1, RGFs 1⎼ 3, TDIF/CLE41 ⎼ and of flg22 complexed with LRR domains of corresponding LRR⎼RKs and co-receptors SERKs have been determined. However, their structures are well not analyzed and characterized in detail. Secondary structure assignments and HELFIT analyses (calculating helix axis, pitch, radius, residues per turn, and handedness) were performed based on the atomic coordinates from their crystal structures. AtPep1 with 23 residues adopts two left handed polyproline helices (PPIIs) with six and four residues. IDA, IDL1, RGFs 1 ⎼ 2, and TDIF/CLE41 with 12 or 13 residues adopt a four residue PPII; RGF3 adopts two PPIIs with four residues. Flg22 with 22 residues also adopts a six residue PPII. The other peptide hormones - PIP, CEP, and CIF - (whose structures are unknown) that are rich in proline or hydroxyproline presumably prefer PPII. The present analysis indicates that PPII helix in the plant small peptide hormones and in flg22 is crucial for recognition of the LRR domains in receptors.