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SCF complexes are the largest family of E3 ubiquitin?protein ligases and mediate the ubiquitination of diverse regulatory and signalling proteins. Here we present the crystal structure of the Cul1?Rbx1?Skp1?F boxSkp2 SCF complex, which shows that Cul1 is an elongated protein that consists of a long stalk and a globular domain. The globular domain binds the RING finger protein Rbx1 through an intermolecular β-sheet, forming a two-subunit catalytic core that recruits the ubiquitin-conjugating enzyme. The long stalk, which consists of three repeats of a novel five-helix motif, binds the Skp1?F boxSkp2 protein substrate-recognition complex at its tip. Cul1 serves as a rigid scaffold that organizes the Skp1?F boxSkp2 and Rbx1 subunits, holding them over 100 Å apart. The structure suggests that Cul1 may contribute to catalysis through the positioning of the substrate and the ubiquitin-conjugating enzyme, and this model is supported by Cul1 mutations designed to eliminate the rigidity of the scaffold. [ABSTRACT FROM AUTHOR]

α-Hemolysin from Staphylococcus aureus assembles from a water-soluble, monomeric species to a membrane-bound heptamer on the surface of target cells, creating water-filled channels that lead to cell death and lysis. Staphylococcus aureus also produces the γ-hemolysin and leukocidin toxins, which function as two component toxins in the disruption and lysis of erythrocytes and leukocytes. Analysis of the aligned sequences of α-hemolysin, γ-hemolysin, and leukocidin in the context of the α-hemolysin heptamer structure supports the conclusion that even though the level of sequence identity between α-hemolysin and the γ-hemolysin and leukocidin toxins is in the so-called twilight zone, the three-dimensional structures of the protomers are probably conserved. By analogy with α-hemolysin, γ-hemolysin and leukocidin may also form oligomeric, transmembrane channels in which an anti-parallel β-barrel constitutes the primary membrane-embedded domain. [ABSTRACT FROM AUTHOR]