Splicing Factor Proline and Glutamine rich (SFPQ) is a multifunctional nuclear protein, implicated in many processes, such as splicing, transcriptional regulation, nuclear retention of mRNA and DNA repair. Along with two other family member proteins – paraspeckle protein component 1 (PSPC1) and Non-POU domain-containing octamer-binding protein (NONO), SFPQ makes up the Drosophila behaviour/human splicing (DBHS) family in humans . Members of the DBHS protein family share a common core region termed the DBHS domain. The DBHS proteins are obligate dimers, forming either homodimers or heterodimers in vitro and emerging evidence suggests that these proteins function predominantly as heterodimers in vivo. Due to the high similarity between DBHS proteins, they share a large functional overlap; the loss of one family protein can be covered by another protein to maintain partial function. Despite this, diseases such as human intellectual disability has been linked to mutations on one specific family protein, indicating the existence of functional diversity between family members.
To characterize the structural determinants of preferential heterodimerisation of human DBHS family protein, SFPQ-containing heterodimers (SFPQ/PSPC1 and SFPQ/NONO) and homodimer were expressed, purified and characterised using analytical ultracentrifugation. In addition, the crystal structure of a SFPQ/PSPC1 heterodimer has been determined to 2.3 Å resolution. As expected from the high-sequence similarity (> 70 %) within the DBHS domain, the structure of a SFPQ/PSC1 heterodimer is highly similar to that of a homodimeric SFPQ. However, a surprisingly 20-fold higher affinity was observed by comparing SFPQ/PSPC1 heterodimer to SFPQ homodimer using analytical ultracentrifugation. By comparing crystal structure of both proteins, subtle differences in the dimerization interfaces are observed, reinforcing the concept that the synergistic effects of subtle variations result in stronger affinity in DBHS heterodimers. In addition, the biological implications of the different affinities among the SFPQ-containing heterodimers and SFPQ homodimer will be discussed.