Sequence-specific RNA-binding protein that binds to the cytoplasmic polyadenylation element (CPE), an uridine-rich sequence element (consensus sequence 5'-UUUUUAU-3') within the mRNA 3'-UTR (PubMed:24990967). RNA binding results in a clear conformational change analogous to the Venus fly trap mechanism (PubMed:24990967). Regulates activation of unfolded protein response (UPR) in the process of adaptation to ER stress in liver, by maintaining translation of CPE-regulated mRNAs in conditions in which global protein synthesis is inhibited (By similarity). Required for cell cycle progression, specifically for cytokinesis and chromosomal segregation (PubMed:26398195). Plays a role as an oncogene promoting tumor growth and progression by positively regulating translation of t-plasminogen activator/PLAT (PubMed:22138752). Stimulates proliferation of melanocytes (PubMed:27857118). In contrast to CPEB1 and CPEB3, does not play role in synaptic plasticity, learning and memory (By similarity). Interacts with TOB1. Expressed in pancreas in islets and ductal cells (at protein level) (PubMed:22138752). Expressed in melanocytes (PubMed:27857118). The 2 RRM domains and the C-terminal region mediate interaction with CPE-containing RNA (PubMed:24990967). The interdomain linker (564-579) acts as a hinge to fix the relative orientation of the 2 RRMs (PubMed:24990967). The ZZ domain (509-566) coordinates 2 Zn ions and is probably implicated in mediating interactions with other proteins in addition to increasing the affinity of the RRMs for the CPEs (By similarity). Unlike in CPEB1, a continuous polar interface is formed between the 2 RRMs (PubMed:24990967). Belongs to the RRM CPEB family.