Two unprecedented natural Aib-peptides with the (Xaa-Yaa-Aib-Pro) motif and an unusual C-terminus: Structures, membrane-modifying and antibacterial properties of pseudokonins KL III and KL VI from the fungus Trichoderma pseudokoningii

Pseudokonins KL III and KL VI are two natural ten-residue peptides, which b oth contain the (Xaa-Yaa-Aib-Pro) motif and exhibit an unusual C-terminus. They have been isolated from the fungus Trichoderma pseudokoningii by inten sive reversed-phase HPLC, beside peptaibols classically C-ended by a beta - amino alcohol. The amino acid sequences and the chemical structures of the C-ends have been determined by the combined use of positive ion LSI-MS and two-dimensional homo- and heteronuclear NMR, including COSY, TOCSY, ROESY, C-13 heteronuclear single quantum correlation (HSQC) and heteronuclear mult iple bond correlation (HMBC). Instead of one of the amino alcohols usually found as C-terminal residue in peptaibols, pseudokonins KL III and KL VI ar e characterized by -Pro-NH2 and cyclo-(Aib-L-Proal) (Proal, prolinal), resp ectively. Such backbone modifications are described here for the first time for peptaibol antibiotics. The unusual cyclo-(Aib-L-Proal) C-terminus is p robably the result of an intramolecular cyclization of the two last Aib and pro residues of a ten-amino acid precursor, via a Proal intermediate. A se condary structure stabilized by -C=O...H-N-hydrogen bonds of the 1 <-- 4 ty pe has been deduced for both peptides from ROESY data, (3)J(NHC>H), couplin gs and amide proton temperature coefficient values. The (Xaa-Yaa-Aib-Pro) b eta -bend ribbon spiral, which has been described for the first time in the case of a 14-residue peptaibol containing three repetitive (Xaa-Yaa-Aib-Pr o) motifs (Segalas G et al. Biopolymers 1999: 50: 71-85) appears to be main tained in the two shortened modified peptides. The beta -bend ribbon struct ure thus appears to be initiated by a single (Xaa-Yaa-Aib-Pro) motif and un affected by the C-terminal modifications. However, the membrane and antibio tic properties of pseudokonins KL III and KL VI, point to the unfavourable effect of both shortening and cyclization of the peptide chain. Copyright ( C) 2000 European Peptide Society and John Wiley gr Sons, Ltd.