Nisin is a cationic polycyclic bacteriocin secreted by some lactic acid bacteria. Nisin has previously been shown to permeabilize liposomes. The interaction of nisin was analyzed with liposomes prepared of the zwitterionic phosphatidylcholine (PC) and the anionic phosphatidylglycerol (PG). Nisin induces the release of 6-carboxyfluorescein and other small anionic fluorescent dyes from PC liposomes in a delta psi-stimulated manner, and not that of neutral and cationic fluorescent dyes. This activity is blocked in PG liposomes. Nisin, however, efficiently dissipates the delta psi in cytochrome c oxidase proteoliposomes reconstituted with PG, with a threshold delta psi requirement of about -100 mV. Nisin associates with the anionic surface of PG liposomes and disturbs the lipid dynamics near the phospholipid polar head group-water interface. Further studies with a novel cationic lantibiotic, epilancin K7, indicate that this molecule penetrates into the hydrophobic carbon region of the lipid bilayer upon the imposition of a delta psi. It is concluded that nisin acts as an anion-selective carrier in the absence of anionic phospholipids. In vivo, however, this activity is likely to be prevented by electrostatic interactions with anionic lipids of the target membrane. It is suggested that pore formation by cationic (type A) lantibiotics involves the local perturbation of the bilayer structure and a delta psi-dependent reorientation of these molecules from a surface-bound into a membrane-inserted configuration.

The homodimeric SecA protein is the peripheral subunit of the preprotein translocase in bacteria. It binds the preprotein and promotes its translocation across the bacterial cytoplasmic membrane by nucleotide modulated coinsertion and deinsertion into the membrane. SecA has two essential nucleotide binding sites (NBS; Mitchell & Oliver, 1993): The high-affinity NBS-I resides in the amino-terminal domain of the protein, and the low-affinity NBS-II is localized at 2/3 of the protein sequence. The nucleotide-bound states of soluble SecA were studied by site directed tryptophan fluorescence spectroscopy, tryptic digestion, differential scanning calorimetry, and dynamic light scattering. A nucleotide-induced conformational change of a carboxy-terminal domain of SecA was revealed by Trp fluorescence spectroscopy. The Trp fluorescence of a single Trp SecA mutant containing Trp775 decreased and increased upon the addition of NBS-I saturating concentrations of ADP or AMP-PNP, respectively. DSC measurements revealed that SecA unfolds as a two domain protein. Binding of ADP to NBS-I increased the interaction between the two domains whereas binding of AMP-PNP did not influence this interaction. When both NBS-I and NBS-II are bound by ADP, SecA seems to have a more compact globular conformation whereas binding of AMP-PNP seems to cause a more extended conformation. It is suggested that the compact ADP-bound conformation resembles the membrane deinserted state of SecA, while the more extended ATP-bound conformation may correspond to the membrane inserted form of SecA.

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