Characterization of a type of beta-bend ribbon spiral generated by the repeating (Xaa-Yaa-Aib-Pro) motif: The solution structure of harzianin HCIX, a14-residue peptaibol forming voltage-dependent ion channels

The three-dimensional solution structure of harzianin HC TX, a peptaibol an tibiotic isolated from the fungus Trichoderma harzianum, was determined usi ng CD, homonuclear, and heteronuclear two-dimensional nmr spectroscopy comb ined with molecular modeling. This 14-residue peptide, Ac Aib(1) Asn(2) Leu (3) Aib(4) Pro(5) Ala(6) Ile(7) Aib(8) Pro(9) Iva(10) Leu(11) Aib(12) Pro(1 3) Leuol(14) (Aib, alpha-aminoisobutyric acid; Iva, isovaline; Leuol, leuci nol), is a main representative of a short-sequence peptaibol class characte rized by an acetylated N-terminus, a C-terminal amino alcohol, and the pres ence of three Aib-L-Pro motifs at positions 4-5, 8-9, and 12-13, separated by two dipeptide units. In spite of a lower number of residues compared to the 18/20-residue peptaibols such as alamethicin, harzianin NC IX exhibits remarkable membrane-perturbing properties. It interacts with phospholipid b ilayers, increasing their permeability and forming voltage-gated ion channe ls through a mechanism slightly differing from that proposed for alamethici n. Sequence-specific H-1- and C-13-nmr assignments and conformational nmr p arameters ((3)J(NHC alpha H) coupling constants, quantitative nuclear Overh auser enhancement data, temperature coefficients of amide and carbonyl grou ps, NH-ND exchange rates) were obtained in methanol solution. Sixty structu res were calculated based on 98 interproton distance restraints and 6 Phi d ihedral angle restraints, using high temperature restrained molecular dynam ics and energy minimization. Thirty-seven out of the sixty generated struct ures were consistent with the nmr data and were convergent. The peptide bac kbone consists in a ribbon of overlapping beta-turns twisted into a continu ous spiral from Asn(2) to Leuol(14) and forming a 26 Angstrom long helix-li ke structure. This structure is slightly amphipathic, with the three Aib-Pr o motifs aligned on the less hydrophobic face of the spiral where the Asn(2 ) sine chain is also present, while the more hydrophobic bulky side drains of leucines, isoleucine, isovaline, and leucinol are located on the concave side. The repetitive (Xaa-Yaa-Aib-Pro) tetrapeptide subunit, making up the peptide sequence, is characterized by four sets of (Phi,Psi) torsional ang les, with the following mean values: Phi(i) = -90 degrees, Psi(i) = -27 deg rees; Phi(i+1) = -98 degrees, Psi(i+1) = -17 degrees; Phi(i+2) = -49 degree s, Psi(i+2) = -50 degrees; Phi(i+3) = -78 degrees, Psi(i+3) = +3 degrees. W e term this particular structure, specifically occurring in the case of (Xa a-Yaa-Aib-Pro)(n) sequences, the (Xaa-Yan-Aib-Pro)-beta-bend ribbon spiral. It is stabilized by 4 --> 1 intramolecular hydrogen bonds and differs from both the canonical 3(IO)-helix made of a succession of type III beta-turns and from the beta-bend ribbon spiral that has been described in the case o f (Aib-Pro)n peptide segments. (C) 1999 John Wiley & Sons, Inc.