Transcriptional coactivator for CREB1 which activates transcription through both consensus and variant cAMP response element (CRE) sites (PubMed:29211348, PubMed:30611118). Acts as a coactivator, in the SIK/TORC signaling pathway, being active when dephosphorylated (PubMed:29211348). Acts independently of CREB1 'Ser-133' phosphorylation (By similarity). Enhances the interaction of CREB1 with TAF4 (By similarity). Regulates the expression of specific CREB-activated genes such as the steroidogenic gene, StAR (By similarity). Potent coactivator of PPARGC1A and inducer of mitochondrial biogenesis in muscle cells (By similarity). Binding, as a tetramer, through its N-terminal region, with the bZIP domain of CREB1 enhances recruitment of TAF4 to the promoter (By similarity). 'Arg-314' in the bZIP domain of CREB1 is essential for this interaction (By similarity). Interacts (when phosphorylated at Ser-162 and Se-273) with 14-3-3 proteins (PubMed:28235073, PubMed:30611118). Interacts with YWHAE (PubMed:30611118). Interacts (when phosphorylated at Ser-391) with phosphatase PP2A catalytic subunit PPP2CA and regulatory subunits PPP2R1A and PPP2R2A (PubMed:30611118). Appears to be mainly nuclear. Translocates to the nucleus following adenylyl cyclase or MAP kinase activation (PubMed:30611118). Expressed in brown adipose tissues. Phosphorylation/dephosphorylation states of Ser-273 are required for regulating transduction of CREB activity (PubMed:29211348). CRTCs/TORCs are inactive when phosphorylated, and active when dephosphorylated at this site (PubMed:29211348). May be phosphorylated at Ser-391 by MAPK3/ERK1 and/or MAPK1/ERK2 or by some cyclin-dependent kinases such as CDK1,CDK2 or CDK5 (PubMed:30611118). Following adenylyl cyclase activation, dephosphorylated at Ser-162 and Ser-273 resulting in its dissociation from 14-3-3 proteins probably promoting CRTC3 translocation into the nucleus (PubMed:30611118). Belongs to the TORC family.