NOPS domain, including C-terminal helical extension region, in the p54nrb/PSF/PSP1 familyAll members in this family contain a DBHS domain (for Drosophila behavior, human splicing), which comprises two conserved RNA recognition motifs (RRM1 and RRM2), also termed RBDs (RNA binding domains) or RNPs (ribonucleoprotein domains), and a charged protein-protein interaction NOPS (NONA and PSP1) domain with a long helical C-terminal extension. The NOPS domain specifically binds to RRM2 domain of the partner DBHS protein via a substantial interaction surface. Its highly conserved C-terminal residues are critical for functional DBHS dimerization while the highly conserved C-terminal helical extension, forming a right-handed antiparallel heterodimeric coiled-coil, is essential for localization of these proteins to subnuclear bodies. PSF has an additional large N-terminal domain that differentiates it from other family members. The p54nrb/PSF/PSP1 family includes 54 kDa nuclear RNA- and DNA-binding protein (p54nrb), polypyrimidine tract-binding protein (PTB)-associated-splicing factor (PSF) and paraspeckle protein 1 (PSP1), which are ubiquitously expressed and are well conserved in vertebrates. p54nrb, also termed NONO or NMT55, is a multi-functional protein involved in numerous nuclear processes including transcriptional regulation, splicing, DNA unwinding, nuclear retention of hyperedited double-stranded RNA, viral RNA processing, control of cell proliferation, and circadian rhythm maintenance. PSF, also termed POMp100, is also a multi-functional protein that binds RNA, single-stranded DNA (ssDNA), double-stranded DNA (dsDNA) and many factors, and mediates diverse activities in the cell. PSP1, also termed PSPC1, is a novel nucleolar factor that accumulates within a new nucleoplasmic compartment, termed paraspeckles, and diffusely distributes in the nucleoplasm. The cellular function of PSP1 remains unknown currently. The family also includes some p54nrb/PSF/PSP1 homologs from invertebrate species. For instance, the Drosophila melanogaster gene no-ontransient A (nonA) encoding puff-specific protein Bj6 (also termed NONA) and Chironomus tentans hrp65 gene encoding protein Hrp65. D. melanogaster NONA is involved in eye development and behavior and may play a role in circadian rhythm maintenance, similar to vertebrate p54nrb. C. tentans Hrp65 is a component of nuclear fibers associated with ribonucleoprotein particles in transit from the gene to the nuclear pore.