Together with PML, this tumor suppressor is a major constituent of the PML bodies, a subnuclear organelle involved in a large number of physiological processes including cell growth, differentiation and apoptosis. Functions as a transcriptional coactivator of ETS1 and ETS2 according to PubMed:11909962. Under certain conditions, it may also act as a corepressor of ETS1 preventing its binding to DNA according to PubMed:15247905. Through the regulation of ETS1 it may play a role in angiogenesis, controlling endothelial cell motility and invasion. Through interaction with the MRN complex it may be involved in the regulation of telomeres lengthening. May also regulate TP53-mediated transcription and through CASP8AP2, regulate FAS-mediated apoptosis. Also plays a role in infection by viruses, including human cytomegalovirus and Epstein-Barr virus, through mechanisms that may involve chromatin and/or transcriptional regulation. Homodimer; isoforms are able to heterodimerize. Interacts with members of the HP1 family of nonhistone chromosomal protein, such as CBX5 and CBX3 via the PxVxL motif. Interacts with ETS1; the interaction is direct and modulates ETS1 transcriptional activity. Interacts with the MRN complex which is composed of two heterodimers RAD50/MRE11 associated with a single NBN; recruits the complex to PML-related bodies. Interacts with HIPK2; positively regulates TP53-dependent transcription. Interacts with CASP8AP2; may negatively regulate CASP8AP2 export from the nucleus to the cytoplasm. Interacts with SUMO1P1/SUMO5 (PubMed:27211601). (Microbial infection) Interacts with Epstein-Barr virus EBNA-LP; this interaction is important for EBNA-LP coactivator activity. (Microbial infection) Interacts with human cytomegalovirus/HHV-5 protein UL123; may play a role in infection by the virus. Differences in the subnuclear localization of the different isoforms seem to exist and may also be cell cycle- and interferon-dependent. Accumulates in the cytoplasm upon FAS activation. Forms a reticulate or track-like nuclear pattern with denser concentrations at the nuclear lamina and surrounding the nucleoli, a pattern reminiscent of heterochromatin-rich regions according to PubMed:11313457. Widely expressed. Sp100-B is expressed only in spleen, tonsil, thymus, mature B-cell line and some T-cell line, but not in brain, liver, muscle or non-lymphoid cell lines. Up-regulated by interferon, retinoic acid, TNF-alpha/TNFA and lipopolysaccharide (at protein level). Up-regulated following heat-shock. The HSR domain is important for the nuclear body targeting as well as for the dimerization. Contains one Pro-Xaa-Val-Xaa-Leu (PxVxL) motif, which is required for interaction with chromoshadow domains. This motif requires additional residues -7, -6, +4 and +5 of the central Val which contact the chromoshadow domain. Sumoylated. Sumoylation depends on a functional nuclear localization signal but is not necessary for nuclear import or nuclear body targeting. Sumoylated. Sumoylated with SUMO1. Sumoylation depends on a functional nuclear localization signal but is not necessary for nuclear import or nuclear body targeting. Sumoylation may stabilize the interaction with CBX5. (Microbial infection) Immediate early protein IE1 of human cytomegalovirus (HHV-5) interferes with the sumoylation of SP100. The major isoform Sp100-A, has a calculated molecular weight of 54 kDa, but exhibits aberrant electrophoretic mobilities, with an apparent molecular weight of 100 kDa. Major isoform.