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 beta-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 A 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.