Multiubiquitin chain receptor involved in modulation of proteasomal degradation. Binds to 'Lys-48'-linked polyubiquitin chains in a length-dependent manner and with a lower affinity to 'Lys-63'-linked polyubiquitin chains. Proposed to be capable to bind simultaneously to the 26S proteasome and to polyubiquitinated substrates and to deliver ubiquitinated proteins to the proteasome. Involved in nucleotide excision repair and is thought to be functional equivalent for RAD23B in global genome nucleotide excision repair (GG-NER) by association with XPC. In vitro, the XPC:RAD23A dimer has NER activity. Can stabilize XPC. (Microbial infection) Involved in Vpr-dependent replication of HIV-1 in non-proliferating cells and primary macrophages. Required for the association of HIV-1 Vpr with the host proteasome. Interacts with XPC; the interaction is suggesting the existence of a functional equivalent variant XPC complex. Interacts with PSMD4 and PSMC5. Interacts with ATXN3 (PubMed:30455355). Interacts with UBQLN2. (Microbial infection) Interacts with HIV-1 Vpr. The ubiquitin-like domain mediates interaction with ATXN3. The ubiquitin-like (UBL) and the UBA (ubiquitin-associated) domains interact intramolecularly in a highly dynamic manner, as each UBA domain competes for an overlapping UBL domain surface. Binding of ubiquitin or proteasome subunit PSMD4 disrupt the UBL-UBA domain interactions and drive RAD23A in to an open conformation. Belongs to the RAD23 family. Extended N-terminus.