The mature 19-amino acid STa heat-stable enterotoxin of E. coli has a preceding peptide of 53 amino acids which contains two domams called Pre (aa I-19) and Pro (aa 20-53) sequences, proposed to be essential for extracellular toxm release by this host. The Pro sequence. however, has been proven not be indispensable for this process since Pro deletion mutants secrete STa. To find out If Pre and/or other unremoved natural STa flanking sequences are responsible for toxin secretlon m those mutants we genetically fused mature STa directly to the leader peptide of the periplasmic E coii heat-labile enterotoxin B-subunit (LTB). ExpressIon of this gene fusion resulted m extracellular secretion of biologically active STa by E. co/i independently of natural STa nelghboring genetlc sequences. Moreover. these results suggest that STa might be able to gain access to the extracellular milieu simply upon its entry mto the E. coli periplasm once guided into this compartment by the LTB leader peptide. To test if extracellular secretIon in this fashion might be extended to other dlsulfide bond-rich small peptides. the 13 ammo acid conotoxin GI and a non-enterotoxlc STa-related decapeptide were cloned. None of the two peptldes was found in culture supernatants, in spite of high structural homology to the toxin. Failure to be secreted most likely leads to degradation as peptides were also not detected in bacterial sonicates. We hypothesize that cysteme-rich peptides must have an amino acid length and/or number of disulfide bridges closer to those in STa for them to follow this toxin secretory pathway in E. colr.
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