POLYANION-INDUCED ALPHA-HELICAL STRUCTURE OF A SYNTHETIC 23-RESIDUE PEPTIDE REPRESENTING THE LYSINE-RICH SEGMENT OF THE N-TERMINAL EXTENSION OF YEAST CYTOPLASMIC ASPARTYL-TRANSFER-RNA SYNTHETASE

Conformational studies were performed on the synthetic tricosapeptide N-acetyl-SKKALKKLQKEQEKQRKKEERAL-amide, representing the highly basic segment (residues 30-52) of the N-terminal extension of yeast cytoplas mic aspartyl-tRNA synthetase. Circular dichroism experiments show that , in aqueous solution at neutral pH, the peptide adopts a random confo rmation. The effects of pH, temperature, addition of trifluoroethanol (TFE), and titration with polyanions on the conformation of the peptid e were studied. In TFE or in the presence of an equimolar concentratio n of (phosphate)(18), the peptide adopts a 100% alpha-helical conforma tion. A partially alpha-helical conformation is induced by (phosphate) (4) or d(pT)(8) (respectively 40% and 35% helical content). Raising th e pH in aqueous solution promotes 75% alpha-helicity, with a transitio n pK of 9.9 reflecting deprotonation of lysine residues. On the basis of these results, nuclear magnetic resonance studies were carried out in TFE as well as in aqueous solution in the presence of (phosphate)(1 8), to determine the structure of the molecule. Complete H-1 resonance assignments were obtained by conventional two-dimensional NMR techniq ues. A total of 138 inter-proton constraints derived from NOESY experi ments were used to calculate the three-dimensional structure by a two- stage distance geometry/simulated annealing procedure. The two deduced structures were highly similar and show that nine cationic residues a re segregated on one face of a helical structure, providing an ideal p olycationic interface for binding to polyanionic surfaces.