A Propensity Scale for Type II Polyproline Helices (PPII): Aromatic Amino Acids in Proline-Rich Sequences Strongly Disfavor PPII Due to Proline-Aromatic Interactions

Type II polyproline helices (PPII) are a fundamental secondary structure of proteins, common in globular and nonglobular regions and important in cellular signaling. We developed a propensity scale for PPII using a host-guest system with sequence Ac-GPPXPPGY-NH2, where X represents any amino acid. We found that proline has the highest PPII propensity, but most other amino acids display significant PPII propensities. The PPII propensity of leucine was the highest of all propensities of non-proline residues. Alanine and residues with linear side chains displayed the next highest PPII propensities. Three classes of residues displayed lower PPII propensities: beta-branched amino acids (Thr, Val, and Ile), short amino acids with polar side chains (Asn, protonated Asp, Ser, Thr, and Cys), and aromatic amino acids (Phe, Tyr, and Trp). tert-Leucine particularly disfavored PPII. The basis of the low PPII propensities of aromatic amino acids in this context was significant cis-trans isomerism, with proline-rich peptides containing aromatic residues exhibiting 45-60% cis amide bonds, due to Pro-cis-Pro-aromatic and aromatic-cis-Pro amide bonds.