UBE2M is the E2 for CRL complexes containing cullin 1, 2, 3 and 4 (Huang et al, 2009; Monda et al, 2013). Interaction between UBE2M and the CUL4A and 4B E3 complex is facilitated by a DCUN1D (also known as DCNL) scaffold protein, of which there are 5 in human cells (Kim et al, 2008; Kurz et al, 2008; Meyer-Schaller et al, 2009; Monda et al, 2013; Keuss et al, 2016). DCUN1D proteins interact with higher affinity to the N-terminally acetylated forms of UBE2F and UBE2M (Scott et al, 2011; Monda et al, 2013). Although each of the 5 DCUN1D proteins appears to interact with most cullin subtypes, specificity may arise through differences in expression and localization, and DCUN1D3 may play a specialized role in sequestering CRL E3 ligase complexes at the cell membrane (Monda et al, 2013; Keuss et al, 2016; Meyer-Schaller et al, 2009; Huang et al, 2014; reviewed in Enchev et al, 2103). Although in this pathway, COMMD proteins and DCUN1D are shown acting sequentially in the activation of the CRL E3 ligase complex, the relationship between these protein families is not totally clear, as DCUN1D proteins have been identified in complexes that also contain the inhibitor CAND1 (Kim et al, 2008; Huang et al, 2014).
CRL4 complexes ubiquitinate target proteins involved in processes such as cell cycle progression, DNA repair and replication, cell growth and metabolism (reviewed in Hannah and Zhou, 2015; Sang et al, 2015). CRL4 complexes are also hijacked by a number of viruses, redirecting the ubiquitin ligase complex to target host proteins and in this way promoting viral propagation (reviewed in Mahon et al, 2014). Note that because many of the key CRL4 ubiquitin targets are nuclear, these complexes are depicted in the nucleus. Cytoplasmic targets have also been identified, however (reviewed in Hannah and Zhou, 2015).