Substrate targeting component of a cullin-RING-based E3 ubiquitin-protein ligase complex RTT101(MMS1-MMS22). RTT101(MMS1-MMS22) promotes fork progression through damaged DNA or natural pause sites by stabilizing replication proteins like the replication fork-pausing complex (FPC) and leading-strand polymerase at stalled replication forks. RTT101(MMS1-MMS22) ubiquitinates the acetylated histones H3K56ac-H4 at lysine residues H3K121, H3K122 and H3K125. Ubiquitination is required for efficient histone deposition during replication-coupled nucleosome assembly, probably by facilitating the transfer of H3-H4 from ASF1 to other chaperones involved in histone deposition