1.D.5 The Alamethicin or Peptaibol Antibiotic Channel-forming (Alamethicin) Family

Alamethicin is a toxic, 20 or 21 amino acid peptide from the fungus, Trichoderma viride, containing nonstandard amino acids α-methyl alanine and α-amino isobutyrate. It adopts helical secondary monomeric and dimeric helix-bend-helix structures that self assemble in membranes into ion conducting helix bundles. It inserts via its N-terminus in response to voltage and laterally aggregates to form funnel-shaped pores of varying sizes. The helices within the bundles are probably oriented with the hydrophilic faces of the amphipathic helices facing inwards to form the channel lining and the hydrophobic side chains interacting with the phosphoplipid membrane matrix. The channels formed conduct ions in a non-saturable fashion, suggesting that transport occurs in a diffusional process without ion binding at discrete sites in the channel. The channels are mildly cation-selective, but mildly anion-selective analogues have been synthesized by substituting a glutamine for a lysine residue at position 18. These observations suggest that long-range electrostatic interactions explain the ion selectivity properties of alamethicin channels. The crystal structure of alamethicin has been determined to 1.5 Å resolution.

Alamethicin was the first isolated and is the best characterized member of the peptaibol class of natural linear depsipeptide antibiotics. These compounds are characterized by acetylated N-termini, a high percentage of α-amino-isobutyrate (AIB), and a C-terminal amino alcohol. Most contain 18-20 residues. Because of their high proportion of AIB, they form α-helical structures. Voltage causes them to autoassociate to form ion channels. The alamethicin pore is of the barrel-stave type consisting of eight alamethicin helices (Qian et al., 2008). Many natural members of the peptaibol family are known and others have been synthesized. Natural members include longibrachins I and II, alamethicins F50 and F30, trichocellins TCAII and TCBII, saturnisporin SAIV, and trichosporin TSBVIa. Their sequences are presented in Cosette et al. (1999).

Trichotoxin_A50E is an 18-residue peptaibol antibiotic which forms multimeric transmembrane channels through self-association. The crystal structure of trichotoxin has been determined at a resolution of 0.9 Å. The trichotoxin sequence contains nine helix-promoting Aib residues, which contribute to the formation of an entirely helical structure that has a central bend of 8-10º located between residues 10-13. Trichotoxin is the first solved structure of the peptaibol family that is all α-helix as opposed to containing part or all 3₁₀-helix. Gln residues in positions 6 and 17 produce a polar face, and are proposed to form the channel lumen. An octameric model channel has been constructed from the crystal structure. It has a central pore of ~4-5 Å radius, a size sufficient to enable transport of ions, with a constricted region at one end, formed by a ring of Gln6 residues. Electrostatic calculations are consistent with it being a cationic channel.

The generalized transport reaction is:

Ion (in) Ion (out)