Drosophila behaviour/human splicing (DBHS) proteins are a family of RNA-binding proteins characterised by two tandem RNA recognition motifs (RRMs), a NOPS domain and a coiled-coil. They mediate functions relating to transcription regulation, transcriptional and post-transcriptional processing and nuclear export. They also bind to the long non-coding RNA NEAT1 to form paraspeckles. Physiologically, these proteins play roles in neuronal development, cell cycle control and innate immune response. Their dysregulation in cancer shows that they have both tumour suppressor and oncogenic roles in various contexts.
In humans, the protein family comprises three members: NONO, PSPC1 and SFPQ. Structural studies show that these proteins form homo- or heterodimers with each other, exchanging interaction partners dynamically to alter their dimerization state to regulate their function. Interestingly, these proteins are functionally redundant in some biological context while in others there is a requirement for a specific protein family member. This leads us to wonder how specificity in function of DBHS proteins is mediated. We first tested all six DBHS dimer combinations for nucleotide binding preferences. Our results show that while all dimer combinations have similar affinity to uridine homoribopolymers, NONO homodimers showed significantly higher affinity to guanosine homoribopolymers. Next, we tested the affinity of NONO homodimers with different G-rich RNA sequences which form secondary G-quadruplex structures. Although NONO homodimers bind to all the G-quadruplexes structures but with significantly different affinity, suggesting that binding affinity may be sequence-dependent.
Where RNA binds to DBHS proteins structurally could provide invaluable information on how they mediate their function and how disease state could be corrected. Although we have shown RRM1 is required for RNA binding, detailed information on the RNA binding location has remained elusive. Small-angle scattering data may provide clues on the binding location and efforts at co-crystallization of DBHS proteins in complex with RNA are continuing.