Involved in ubiquitination and subsequent proteasomal degradation of target proteins. Together with CUL1, RBX1 and a F- box protein, it forms a SCF E3 ubiquitin ligase complex. The functional specificity of this complex depends of the type of F- box protein. In the SCF complex, it serves as an adapter that links the F-box protein to CUL1 (By similarity).