Functions as an E3-type small ubiquitin-like modifier (SUMO) ligase (By similarity). May play a role as a transcriptional coregulator in various cellular pathways (By similarity). Negatively regulates induction of interferon phi 1 (ifnphi1) mediated by mavs and ticam1/trif. Also inhibits ifnphi1-mediated activation of the interferon-stimulated genes (ISGs) pkz and cd40, and to a lesser extent rsad2 and isg15 (PubMed:22345667). May inhibit ticam1/trif-mediated activation of NF-kappa-B (PubMed:22345667). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein sumoylation. Highly expressed in spleen, liver, and brain. Expressed at lower levels in heart, intestine, kidney, gill, skin, and muscle. Highly expressed in early development (6 hours to 24 hours post fertilization), with lower expression thereafter. Up-regulated in response to polyinosinic-polycytidylic acid (poly(I:C)), and interferon phi 1 (ifnphi1). The LXXLL motif is a coregulator signature that is essential for transcriptional corepression. Sumoylated. Lys-35 is the main site of sumoylation. Belongs to the PIAS family.