Transcriptional repressor mainly required for germinal center (GC) formation and antibody affinity maturation which has different mechanisms of action specific to the lineage and biological functions. Forms complexes with different corepressors and histone deacetylases to repress the transcriptional expression of different subsets of target genes. Represses its target genes by binding directly to the DNA sequence 5'-TTCCTAGAA-3' (BCL6-binding site) or indirectly by repressing the transcriptional activity of transcription factors. In GC B-cells, represses genes that function in differentiation, inflammation, apoptosis and cell cycle control, also autoregulates its transcriptional expression and up-regulates, indirectly, the expression of some genes important for GC reactions, such as AICDA, through the repression of microRNAs expression, like miR155. An important function is to allow GC B-cells to proliferate very rapidly in response to T-cell dependent antigens and tolerate the physiological DNA breaks required for immunglobulin class switch recombination and somatic hypermutation without inducing a p53/TP53-dependent apoptotic response. In follicular helper CD4(+) T-cells (T(FH) cells), promotes the expression of T(FH)-related genes but inhibits the differentiation of T(H)1, T(H)2 and T(H)17 cells. Also required for the establishment and maintenance of immunological memory for both T- and B-cells. Suppresses macrophage proliferation through competition with STAT5 for STAT-binding motifs binding on certain target genes, such as CCL2 and CCND2. In response to genotoxic stress, controls cell cycle arrest in GC B-cells in both p53/TP53-dependedent and -independent manners. Besides, also controls neurogenesis through the alteration of the composition of NOTCH-dependent transcriptional complexes at selective NOTCH targets, such as HES5, including the recruitment of the deacetylase SIRT1 and resulting in an epigenetic silencing leading to neuronal differentiation. Homodimer. Interacts (via BTB domain) with the corepressors BCOR, NCOR1 and SMRT/NCOR2; the interactions are direct. Forms preferably ternary complexes with BCOR and SMRT/NCOR2 on target gene promoters but, on enhancer elements, interacts with SMRT/NCOR2 and HDAC3 to repress proximal gene expression. Interacts with histone deacetylases HDAC2, HDAC5 and HDAC9 (via the catalytic domain). Interacts with ZBTB7 and BCL6B. Interacts with SCF(FBXO11) complex; the interaction is independent of phosphorylation and promotes ubiquitination. Interacts (when phosphorylated) with PIN1; the interaction is required for BCL6 degradation upon genotoxic stress. Interacts with ZBTB17; inhibits ZBTB17 transcriptional activity. Interacts with CTBP1, autoinhibits its transcriptional expression. Interacts with NOTCH1 NCID and SIRT1; leads to a epigenetic repression of selective NOTCH1-target genes. Interacts (nor via BTB domain neither acetylated) with the NuRD complex components CHD4, HDAC1, MBD3 and MTA3; the interaction with MTA3 inhibits BCL6 acetylation and is required for BCL6 transpriptional repression. Expressed at least in germinal center B-cells of spleen. Detected in the cerebral cortex from 12.5 dpc until birth, with highest levels in the frontal and parietal parts of the neocortex than the occipital parts. Interaction with corepressors through the BTB domain is needed to facilitate the rapid proliferation and survival of GC B-cells but is not involved in the T(FH) formation and BCL6-mediated suppression of T(H)2 and T(H)17 differentiation required for GC formation. Phosphorylated by MAPK1 in response to antigen receptor activation at Ser-334 and Ser-344. Phosphorylated by ATM in response to genotoxic stress. Phosphorylation induces its degradation by ubiquitin/proteasome pathway. Polyubiquitinated. Polyubiquitinated by SCF(FBXO11), leading to its degradation by the proteasome. Ubiquitinated by the SCF(FBXL17) complex, leading to its degradation by the proteasome: ubiquitination by the SCF(FBXL17) complex takes place when aberrant BTB domain dimers are formed. Acetylated at Lys-380 by EP300 which inhibits the interaction with NuRD complex and the transcriptional repressor function. Deacetylated by HDAC- and SIR2-dependent pathways. More than 50% of lethality by 6 weeks of age. Mice have infiltrates of inflammatory cells in their lungs, as well as multinodular lesions with eosinophil infiltrations into the spleen, significantly more T(H)2 and T(H)17 cells and up-regulated levels of inflammtaroy chemokines in macrophages, but, express low levels of memory CD8(+) T-cells and, in spleen, GC B and T(FH) cells are both undetectable. B-cells express 10-fold lower levels of surface IgM than control littermates and macrophages divide faster. From 12.5 dpc to at least 21 days after birth, animals have reduced size of the cerebral hemispheres and a reduced thickness of the frontal and parietal cortex with all the cortical layers affected. At 12.5 and 15.5 dpc, marked reduction of cell-cyle exit indicating defective transition from neural progenitor Cells to postmitotic neurons.