HELIX FORMATION IN MODEL PEPTIDES BASED ON NUCLEOLIN TPAKK MOTIFS

The structures formed by peptide models of the N-terminal domain of th e nucleolar protein nucleolin were studied by CD and nmr. The sequence s of the peptides are based on the putative nucleic acid binding seque nce motif TPAKK. The peptides TP1 and TP2 have the sequence acetyl-G(A TPAKKAA)(n)G-amide, with n = 1 and 2, respectively. CD measurements in dicate structural changes in both peptides when the lysine side chains are uncharged by increasing the pH or acetylation of the side-chain a mines. When trifluoroethanol (TFE) is added, more extensive structural changes are observed, resembling helical structure based on nmr nucle ar Overhauser effect (NOE) and C-alpha proton chemical shift changes, and CD spectra. The structure formed in 0.5M NaClO4 as observed by nmr is similar to that when the lysine side chains are acetylated, due pr esumably to interactions of perchlorate ion with side-chain charges on lysines. The helical structure observed in TPAKK motifs may be stabil ized via N-capping interactions involving threonine. The structures ob served in TFE suggest that the Thr-Pro sequence initiates short helica l segments in TPAKK motifs, and these helical structures might interac t with nucleic acids, presumably via interactions between lysines and threonines of nucleolin. (C) 1995 John Wiley & Sons, Inc.