A MOLTEN-GLOBULE MEMBRANE-INSERTION INTERMEDIATE OF THE PORE-FORMING DOMAIN OF COLICIN-A

THE 'molten' globular conformation of a protein is compact with a native secondary structure but a poorly defined tertiary structure 1,2. Molten globular states are intermediates in protein folding and unfolding 3-5 and they may be involved in the translocation or insertion of proteins into membranes 6. Here we investigate the membrane insertion of the pore-forming domain of colicin A, a bacteriocin that depolarizes the cytoplasmic membrane of sensitive cells 7-9. We find that this pore-forming domain, the insertion of which depends on pH (refs 10, 11), undergoes a native to molten globule transition at acidic pH. The variation of the kinetic constant of membrane insertion of the protein into negatively charged lipid vesicles as a function of the interfacial pH correlates with the appearance of the acidic molten globular state, indicating that this state could be an intermediate formed during the insertion of colicin A into membranes.