Sequence-specific RNA-binding protein which acts as a translational repressor in the basal unstimulated state but, following neuronal stimulation, acts as a translational activator (By similarity). In contrast to CPEB1, does not bind to the cytoplasmic polyadenylation element (CPE), a uridine-rich sequence element within the mRNA 3'-UTR, but binds to a U-rich loop within a stem-loop structure (By similarity). Required for the consolidation and maintenance of hippocampal-based long term memory (By similarity). In the basal state, binds to the mRNA 3'-UTR of the glutamate receptors GRIA2/GLUR2 mRNA and negatively regulates their translation (By similarity). Also represses the translation of DLG4, GRIN1, GRIN2A and GRIN2B (By similarity). When activated, acts as a translational activator of GRIA1 and GRIA2 (By similarity). In the basal state, suppresses SUMO2 translation but activates it following neuronal stimulation (By similarity). Binds to the 3'-UTR of TRPV1 mRNA and represses TRPV1 translation which is required to maintain normal thermoception (By similarity). Binds actin mRNA, leading to actin translational repression in the basal state and to translational activation following neuronal stimulation (By similarity). Negatively regulates target mRNA levels by binding to TOB1 which recruits CNOT7/CAF1 to a ternary complex and this leads to target mRNA deadenylation and decay (PubMed:21336257). In addition to its role in translation, binds to and inhibits the transcriptional activation activity of STAT5B without affecting its dimerization or DNA-binding activity. This, in turn, represses transcription of the STAT5B target gene EGFR which has been shown to play a role in enhancing learning and memory performance (PubMed:20639532). In contrast to CPEB1, CPEB2 and CPEB4, not required for cell cycle progression (PubMed:26398195). Following synaptic activity, forms amyloid-like oligomers (By similarity). Aggregation requires an intact actin cytoskeleton (By similarity). Interacts with STAT5B; this inhibits STAT5B-mediated transcriptional activation (PubMed:20639532). Interacts with E3 ubiquitin-protein ligase NEURL1; this leads to monoubiquitination and activation of CPEB3 (By similarity). Interacts with CAPN2; this leads to cleavage of CPEB3 (By similarity). Interacts (via C-terminal RNA-binding region) with TOB1; TOB1 also binds CNOT7/CAF1 and recruits it to CPEB3 to form a ternary complex (PubMed:21336257). Interacts with SUMO-conjugating enzyme UBC9 (By similarity). Interacts with IPO5; the interaction is enhanced in a RAN-regulated manner following neuronal stimulation and mediates CPEB3 nuclear import (PubMed:22730302). Interacts with exportin XPO1/CRM1 (PubMed:22730302). Predominantly cytoplasmic in unstimulated neurons but translocates to the nucleus following neuronal stimulation (PubMed:20639532, PubMed:22730302). Nuclear import is mediated by importin IPO5 (By similarity). The N-terminal Gln-rich region is required for the formation of amyloid-like oligomers and for the stability of long-term potentiation and spatial memory. Activated by NEURL1-mediated monoubiquitination, resulting in the growth of new dendritic spines and increased levels of GRIA1 and GRIA2. NEURL1-mediated monoubiquitination facilitates synaptic plasticity and hippocampal-dependent memory storage. Under basal unstimulated conditions when CPEB3 is mainly unaggregated, sumoylated and acts as a translational repressor. Following neuronal stimulation, becomes desumoylated and aggregated which is required for the translation of mRNA targets and for dendritic filopodia formation. Following neuronal stimulation, cleaved by CAPN2 which abolishes its translational repressor activity, leading to translation of CPEB3 target mRNAs. Phosphorylation is enhanced by neuronal stimulation. The CPEB3 gene contains an intron-encoded self-cleaving ribozyme which is structurally and biochemically related to human hepatitis delta virus ribozymes and which may play a role in the regulation of CPEB3 translation (PubMed:16990549). A polymorphism in the ribozyme sequence which influences cleavage activity of the ribozyme may play a role in episodic memory with carriers of a rare C allele-containing ribozyme showing significantly poorer memory recall performance than T allele carriers (PubMed:19503753). Belongs to the RRM CPEB family. Tenacious memory - Issue 185 of November 2016