1.D.2 The Channel-forming Syringomycin (Syringomycin) Family
Syringomycin is an amphipathic, cyclic lipodepsipeptide which inserts to biological and artificial lipid bilayers to form cation (K+, H+, Ca2+) permeable channels. Monomers aggregate to form a voltage-dependent pore complex which may be a homohexamer. The channel may catalyze K+: H+ antiport under certain conditions. It is produced by the plant pathogen, Pseudomonas syringae and targets host plants, inducing necrosis. The channel radius is about 1 nm, sufficient to allow the release of nutrients from the plant cell which then can be used by the bacteria. In animals, the pore-forming activity of syringomycins renders it hemolytic.
Syringomycin is produced on a multienzyme complex by a thiotemplate mechanism as originally described for peptide antibiotics produced by Bacillus, Streptomyces and filamentous fungi. It contains D-amino acids and other non-proteinogenic amino acids. Threonine is the precursor for both 2,3-dehydroaminobutyrate and 4-chlorothreonine, while aspartate is modified to give 2,4-diaminobutyrate and 3-hydroxyaspartate. The 3-hydroxy fatty acid tail is derived from hydroxyalkanoates.
The generalized transport reaction catalyzed by syringomycin is:
cations (in) 
cations (out)