Sequence-specific RNA-binding protein that regulates mRNA cytoplasmic polyadenylation and translation initiation (PubMed:14697205). In its amyloid-like oligomeric form, acts as a translational activator (PubMed:14697205, PubMed:21270333). Up-regulated at activated synapses where it is required for the late phase of memory-related long-term synaptic facilitation (PubMed:14697206). Sustained CPEB-mediated protein synthesis for at least 72 hours after neuronal stimulation is required for the persistance of synaptic facilitation (PubMed:18817739). Activates translation of atypical protein kinase C (PubMed:26095361). Required for the relocalization of syntaxin mRNA to the axon hillock from the opposite pole in the cell body of sensory neurons during long-term facilitation (PubMed:16687512). Exists in 2 different conformational states, a soluble alpha-helix-rich form and an insoluble beta-sheet-rich amyloid form (PubMed:23435382). Stable self-sustaining active amyloid-like oligomers are formed upon neuronal stimulation and contribute to long-term facilitation in sensory neurons (PubMed:20144764). More than 80% of the soluble form is found in high molecular weight oligomers, the majority of which have the size of a decamer or larger oligomer (PubMed:23435382). The insoluble form binds RNA with significantly greater affinity than the soluble form (PubMed:23435382). A mechanical pulling force can facilitate the alpha-helix to beta-sheet transition by lowering the free energy barrier between the two forms and interactions with the cytoskeleton may provide the necessary mechanical force (PubMed:27091989). In sensory neurons, detected in cytoplasmic granules composed of proteins and RNA. Isoform 1: Enriched in nervous tissue. Isoform 2: Predominates in ovotestis and is only weakly expressed in nervous tissue. By neuronal stimulation with serotonin (at protein level) (PubMed:14697206). MicroRNA-22 (miR-22) binds to the 3'-UTR of CPEB mRNA and inhibits its translation while serotonin triggers down-regulation of miR22, thereby up-regulating CPEB expression (PubMed:26095361). The N-terminal Gln-rich domain can confer two distinct functional states, a dominant active prion-like aggregated state and a soluble state. Belongs to the RRM CPEB family.