CRYSTAL-STRUCTURE OF [LEU1]ZERVAMICIN, A MEMBRANE ION-CHANNEL PEPTIDE - IMPLICATIONS FOR GATING MECHANISMS

Structures in four different crystal forms of [Leu1]zervamicin (zervamicin Z-L, Ac-Leu-Ile-Gln-Iva-Ile5-Thr-Aib-Leu-Aib-Hyp10-Gln-Aib-Hyp-Aib-Pro15-Phol, where Iva is isovaline, Aib is alpha-amino isobutyric acid, Hyp is 4-hydroxyproline, and Phol is phenylalaninol), a membrane channel-forming polypeptide from Emericellopsis salmosynnemata, have been determined by x-ray diffraction. The helical structure is amphiphilic with all the polar moieties on the convex side of the bent helix. Helices are bent at Hyp10 from almost-equal-to 30-degrees to almost-equal-to 45-degrees in the different crystal forms. In all crystal forms, the peptide helices aggregate in a similar fashion to form water channels that are interrupted by hydrogen bonds between N-epsilon-H(Gln11) and O-delta(Hyp10) of adjacent helices. The Gln11 side chain is folded in an unusual fashion in order to close the channel. Space is available for an extended conformation for Gln11, in which case the channel would be open, suggesting a gating mechanism for cation transport. Structural details are presented for one crystal form derived from methanol/water solution: C85H140N18O22.10H2O, space group P2(1), a = 23.068(6) angstrom, b = 9.162(3) angstrom, c = 26.727(9) angstrom, beta = 108.69(2)degrees (standard deviation of last digit is given in parentheses); overall agreement factor R = 10.1% for 5322 observed reflections [[F(o)[ > 3-sigma(F)]; resolution, 0.93 angstrom.