Transcriptional coactivator for CREB1 which activates transcription through both consensus and variant cAMP response element (CRE) sites. Acts as a coactivator, in the SIK/TORC signaling pathway, being active when dephosphorylated and acts independently of CREB1 'Ser-133' phosphorylation. Enhances the interaction of CREB1 with TAF4. Regulates the expression of specific CREB-activated genes such as the steroidogenic gene, StAR. Potent coactivator of PPARGC1A and inducer of mitochondrial biogenesis in muscle cells. Also coactivator for TAX activation of the human T-cell leukemia virus type 1 (HTLV-1) long terminal repeats (LTR). Binding, as a tetramer, through its N-terminal region, with the bZIP domain of CREB1 enhances recruitment of TAF4 to the promoter (PubMed:14506290). 'Arg-314' in the bZIP domain of CREB1 is essential for this interaction (PubMed:14506290). Interacts (when phosphorylated at Ser-162 and Se-273) with 14-3-3 proteins (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). Interacts, via the N-terminal with the ankyrin repeats of BCL3, to form a complex with CREB1 on CRE and TxRE responsive elements and represses HTLV-1 LTR-mediated transcription (PubMed:17644518). (Microbial infection) Interacts with HTLV-1 protein Tax; this interaction enhances tax transcriptional activity. Appears to be mainly nuclear (PubMed:15454081). Translocates to the nucleus following adenylyl cyclase or MAP kinase activation (PubMed:30611118). Predominantly expressed in B and T lymphocytes. Highest levels in lung. Also expressed in brain, colon, heart, kidney, ovary, and prostate. Weak expression in liver, pancreas, muscle, small intestine, spleen and stomach. Phosphorylation/dephosphorylation states of Ser-273 are required for regulating transduction of CREB activity (By similarity). CRTCs/TORCs are inactive when phosphorylated, and active when dephosphorylated at this site (By similarity). 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.