| Abstract
| - Carnobacteriocin B2 (CbnB2), a type IIa bacteriocin, is a 48 residue antimicrobial peptidefrom the lactic acid bacterium Carnobacterium pisicola LV17B. Type IIa bacteriocins have a conservedYGNGVXC sequence near the N-terminus and usually contain a disulfide bridge. CbnB2 seemed to beunique in that its two cysteines (Cys9 and Cys14) could be isolated as free thiols [Quadri et al. (1994) J.Biol. Chem. 26, 12204−12211]. To establish the structural consequences of the presence or absence of adisulfide bridge and to investigate if the YGNGVXC sequence is a receptor-binding motif [Fleury et al.(1996) J. Biol. Chem. 271, 14421−14429], the three-dimensional solution structure of CbnB2 wasdetermined by two-dimensional 1H nuclear magnetic resonance (NMR) techniques. Mass spectroscopicand thiol modification experiments on CbnB2 and on model peptides, in conjunction with activitymeasurements, were used to verify the redox status of CbnB2. The results show that CbnB2 readily formsa disulfide bond and that this peptide has full antimicrobial activity. NMR results indicate that CbnB2 intrifluoroethanol (TFE) has a well-defined central helical structure (residues 18−39) but a disordered Nterminus. Comparison of the CbnB2 structure with the refined solution structure of leucocin A (LeuA),another type IIa bacteriocin, indicates that the central helical structure is conserved between the two peptidesdespite differences in sequence but that the N-terminal structure (a proposed receptor binding site) is not.This is unexpected because LeuA and CbnB2 exhibit >66% sequence identity in the first 24 residues.This suggests that the N-terminus, which had been proposed [Fleury et al. (1996) J. Biol. Chem. 271,14421-14429] to be a receptor binding site of type IIa bacteriocins, may not be directly involved and thatrecognition of the amphiphilic helical portion is the critical feature.
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