Core component of multiple cullin-RING-based E3 ubiquitin-protein ligase complexes (CRLs), which mediate the ubiquitination of target proteins. Recruits the E2 ubiquitin-conjugating enzyme CDC34/UBC3 to the complex and brings it into close proximity to the substrate. Also stimulates CDC34/UBC3 autoubiquitination and promotes the neddylation of CDC53 and RTT101. Component of the SCF(CDC4) ubiquitin ligase required for ubiquitination of the cyclin-dependent kinase inhibitor SIC1 and for the G1-to-S phase transition. Component of the RTT101(MMS1-MMS22) ubiquitin ligase that promotes fork progression through damaged DNA or natural pause sites. Component of the CRL3(ELA1) ubiquitin ligase required for ubiquitination of RPB1, the largest subunit of RNA polymerase II (Pol II), which targets Pol II for proteasomal degradation in DNA-damaged cells. Protein modification; protein ubiquitination. Component of multiple cullin-RING ligases (CRLs) composed of 4 subunits: the RING protein HRT1, a cullin, a linker protein, and one of many alternative substrate receptors. Component of SCF E3 ubiquitin ligase complexes containing the cullin CDC53, the linker protein SKP1/CBF3D, and substrate receptors containing F-box motifs like DAS1 or GRR1. Component of RTT101(MMS1) E3 ubiquitin ligase complexes containing the cullin RTT101, the linker protein MMS1, and substrate receptors belonging to a protein family described as DCAF (DDB1- and CUL4-associated factor) like MMS22. Component of CRL3 E3 ubiquitin ligase complexes containing the cullin CUL3, the linker protein ELC1, and substrate receptors containing SOCS-box motifs like ELA1. Interacts with CDC53, CUL3, RTT101, CDC4 and CDC34/UBC3. The RING-type zinc finger domain is essential for ubiquitin ligase activity. Belongs to the RING-box family. It is uncertain whether Met-1 or Met-8 is the initiator.