The structure of human SFPQ reveals a coiled-coil mediated polymer essential for functional aggregation in gene regulation.

SFPQ, (a.k.a. PSF), is a human tumor suppressor protein that regulates many important functions in the cell nucleus including coordination of long non-coding RNA molecules into nuclear bodies. Here we describe the first crystal structures of Splicing Factor Proline and Glutamine Rich (SFPQ), revealing structural similarity to the related PSPC1/NONO heterodimer and a strikingly extended structure (over 265 Å long) formed by an unusual anti-parallel coiled-coil that results in an infinite linear polymer of SFPQ dimers within the crystals. Small-angle X-ray scattering and transmission electron microscopy experiments show that polymerization is reversible in solution and can be templated by DNA. We demonstrate that the ability to polymerize is essential for the cellular functions of SFPQ: disruptive mutation of the coiled-coil interaction motif results in SFPQ mislocalization, reduced formation of nuclear bodies, abrogated molecular interactions and deficient transcriptional regulation. The coiled-coil interaction motif thus provides a molecular explanation for the functional aggregation of SFPQ that directs its role in regulating many aspects of cellular nucleic acid metabolism.

Structure of the heterodimer of human NONO and paraspeckle protein component 1 and analysis of its role in subnuclear body formation.

Proteins of the Drosophila behavior/human splicing (DBHS) family include mammalian SFPQ (PSF), NONO (p54nrb), PSPC1, and invertebrate NONA and Hrp65. DBHS proteins are predominately nuclear, and are involved in transcriptional and posttranscriptional gene regulatory functions as well as DNA repair. DBHS proteins influence a wide gamut of biological processes, including the regulation of circadian rhythm, carcinogenesis, and progression of cancer. Additionally, mammalian DBHS proteins associate with the architectural long noncoding RNA NEAT1 (Menε/ß) to form paraspeckles, subnuclear bodies that alter gene expression via the nuclear retention of RNA. Here we describe the crystal structure of the heterodimer of the multidomain conserved region of the DBHS proteins, PSPC1 and NONO. These proteins form an extensively intertwined dimer, consistent with the observation that the different DBHS proteins are typically copurified from mammalian cells, and suggesting that they act as obligate heterodimers. The PSPC1/NONO heterodimer has a right-handed antiparallel coiled-coil that positions two of four RNA recognition motif domains in an unprecedented arrangement on either side of a 20-Å channel. This configuration is supported by a protein:protein interaction involving the NONA/paraspeckle domain, which is characteristic of the DBHS family. By examining various mutants and truncations in cell culture, we find that DBHS proteins require an additional antiparallel coiled-coil emanating from either end of the dimer for paraspeckle subnuclear body formation. These results suggest that paraspeckles may potentially form through self-association of DBHS dimers into higher-order structures.