Structural model of the ubiquitin-E2-Cul3-Rbx1-BBK complex. The complex forms a dimer by the self-association of the BTB domain in the BBK protein. The approximate position of the two-fold axis is indicated. Each full-length BBK protein is shown in red, with the BTB dimer shown in the darkest shading in surface representation, the two BACK domains in pink surface, and the two Kelch β-propellers shown in pink cartoon representation. The Cul3 homology model is shown in green cartoon representation, Rbx1 is in gray cartoon representation, E2 Ubch7 is in yellow cartoon representation, and ubiquitin is shown as a blue surface. Stars indicate the position associated with substrate binding . Depth cuing is used to indicate distances in the plane of the page, such that the diffuse colors are most distant to the viewer than the intense colors.