The molten globule intermediate for protein insertion or translocation through membranes

The cytolytic toxin aerolysin: from the soluble form to the transmembrane channel

Aerolysin is a cytolytic toxin which forms channels in the plasma membranes of eucaryotic cells. The protein is secreted by Aeromonas hydrophila as an inactive protoxin. Its stability and water solubility are conferred by its ability to dimerize. Maturation of the protein occurs through proteolytic removal of a C-terminal peptide outside the secreting cell. Although the aerolysin which is so produced is still a dimer, it then has the ability to oligomerize. The oligomer is the active form of the toxin, capable of forming the transmembrane channels that disrupt cells. We review here the present knowledge about the structure of aerolysin in relation to the various steps in channel formation.

All in the family: the toxic activity of pore-forming colicins

Colicins are unusual bacterial toxins because they are directed against close relatives of the producing strain. They kill their targets in one of three distinct ways; via a ribonuclease or deoxyribonuclease activity or by forming pores in the target cell's membrane. This review deals with the steps involved in pore-forming colicin activity including, initial synthesis of the toxin, toxin release, receptor binding, translocation across the periplasm and pore formation in the cytoplasmic membrane. Special reference is made to the role of colicin in vivo, the structural changes occurring during pore formation and the role of the immunity protein.

Three-dimensional structure of (1-71)bacterioopsin solubilized in methanol/chloroform and SDS micelles determined by 15N-1H heteronuclear NMR spectroscopy

Spatial structures of a chymotryptic fragment C2 (residues 1-71) of bacterioopsin from Halobacterium halobium, solubilized in a mixture of methanol/chloroform (1:1, by vol.) and 0.1 M 2HCO2NH4, or in perdeuterated sodium (2H)dodecyl sulfate (SDS) micelles in the presence of perdeuterated (2,2,2-2H)trifluoroethanol, were determined by two-dimensional and three-dimensional heteronuclear 15N-1H NMR techniques. The influence of (2,2,2-2H)trifluoroethanol on the conformational dynamics of C2 in micelles and the effect of the salt (organic mixture) were studied. Under the best conditions, 1H and 15N resonances of 15N-uniformly enriched protein were assigned in both milieus by homonuclear two-dimensional NOE (NOESY) and two-dimensional total-correlated (TOCSY) spectra and heteronuclear three-dimensional NOESY-multiple-quantum-correlation (HMQC) and TOCSY-HMQC spectra. 651 (organic mixture) and 520 (micelles) interproton-distance constraints, derived from volumes of cross-peaks in two-dimensional NOESY and three-dimensional NOESY-HMQC spectra, along with deuterium exchange rates of amide groups measured in both milieus and 51 HN-C alpha H coupling constants obtained in the case of the organic mixture, were used in the construction of C2 spatial structures. Obtained structures are similar in both milieus and have two right-handed alpha-helical regions stretching from Pro8 to Met32 and Phe42 to Tyr64 (organic mixture), and from Pro8 to Met32 and Ala39 to Leu62 (micelles). In micelles, the second alpha helix is terminated by C-cap Gly63, adopting a conformation characteristic of a left-handed helix. Residues Gly65 to Thr67 from the turn of a right-handed helix. In the isotropic medium of the organic mixture, the C-terminal region of residues 65-71 lacks an ordered structure. Torsion angles chi 1 were unequivocally determined for 18 alpha-helical residues in both milieus. In the isotropic organic mixture and anisotropic micellar system, C2 remains a compact structure with a characteristic size of 3.0-3.5 nm. C2 seems to be present in at least two conformational states, packed and unpacked. Using NMR data, along with the electron cryomicroscopy model of bacteriorhodopsin [Henderson, R., Baldwin, J. M., Ceska, T. A., Zemlin, F., Beckman, E. & Downing, K. H. (1990) J. Mol. Biol. 213, 899-929], we suggested a model for the conformation of C2 in this putative close-packed state. However, no NOE contact between alpha helices was found in either milieu.