Staphylococcal gamma-hemolysins are bicomponent toxins forming a protein family with leucocidins and alpha-toxin. Two active toxins (AB and CB) can be formed combining one of the class-S components, HlgA or HlgC, with the class-F component HlgB. These two gamma-hemolysins form pores with marked similarities to alpha-toxin in terms of conductance, nonlinearity of the current-voltage curve, and channel stability in the open state. AB and CB pores, however, are cation-selective, whereas alpha-toxin is anion-selective. gamma-Hemolysins' pores are hetero-oligomers formed by three or four copies of each component (indicated as 3A3B and 3C3B or 4A4B and 4C4B). Point mutants located on a beta-strand of the class-S component that forms part of the protomer-protomer contact region can prevent oligomer assembly. Interestingly, these mutants inhibit growth of pores formed not only by their natural components but also by nonstandard components. This lead to the hypothesis that mixed ABC pores could also be formed. By studying the conductance of pores, assembled in the presence of all three components (in different ratios), it was observed that the magnitudes expected for mixed pores were, indeed, present. We conclude that the gamma-hemolysin/leucocidin bicomponent toxin family may form a larger than expected number of active toxins by cross-combining various S and F components.

Staphylococcus aureus strains causing human pathologies produce several toxins, including a pore‐forming protein family formed by the single‐component α‐hemolysin and the bicomponent leukocidins and γ‐hemolysins. The last comprise two protein elements, S and F, that co‐operatively form the active toxin. α‐Hemolysin is always expressed by S. aureus strains, whereas bicomponent leukotoxins are more specifically involved in a few diseases. X‐ray crystallography of the α‐hemolysin pore has shown it is a mushroom‐shaped, hollow heptamer, almost entirely consisting of β‐structure. Monomeric F subunits have a very similar core structure, except for the transmembrane stem domain which has to refold during pore formation. Large deletions in this domain abolished activity, whereas shorter deletions sometimes improved it, possibly by removing some of the interactions stabilizing the folded structure. Even before stem extension is completed, the formation of an oligomeric pre‐pore can trigger Ca 2+ ‐mediated activation of some white cells, initiating an inflammatory response. Within the bicomponent toxins, γ‐hemolysins define three proteins (HlgA, HlgB, HlgC) that can generate two toxins: HlgA+HlgB and HlgC+HlgB. Like α‐hemolysin they form pores in planar bilayers with similar conductance, but opposite selectivity (cation instead of anion) for the presence of negative charges in the ion pathway. γ‐Hemolysin pores seem to be organized as α‐hemolysin, but should contain an even number of each component, alternating in a 1:1 stoichiometry.

After an introductory survey of the X-ray resonant anisotropy, we present a resonant X-ray method to observe the thermal-motion-induced (TMI) and point-defect-induced (PDI) distortion of electronic states of atoms.This novel method uses an idea that, in general, the local atomic environment becomes less symmetric owing to point defects and/or thermal vibrations of atoms in crystals.