Engineering a protein homodimer from a heterodimer: A chimeric DBHS protein.

Drosophila behavior/human splicing (DBHS) proteins are involved at nearly every stage of the RNA lifecycle. There are three paralogues in mammals-SFPQ, NONO and PSPC1-which form both homodimer and heterodimers. It is likely that the non-redundant and overlapping roles of the different DBHS paralogues emerge from their dynamic and combinatorial nature; that is, their ability to mix and match dimer partners. A strong preference for heterodimerisation over homodimerisation has been demonstrated. However, the underlying molecular determinants of DBHS partner preference are poorly understood. This study describes the design, and biochemical and structural characterization of a chimeric DBHS protein designed to be a homodimer with an interface that emulates a DBHS heterodimer. In vitro experiments show that this chimera indeed remains as a homodimer even when in the presence of a native DBHS protein that prefers heterodimerisation. The ease of purification, production of high-quality crystals for X-ray diffraction, and perfect crystallographic symmetry of this engineered homodimer presents this as an attractive approach for future structural studies on DBHS proteins. This ability to engineer partner preference also highlights the potential for DBHS proteins to serve as versatile building blocks in synthetic biology. Furthermore, comparison of the dimer interface of this engineered homodimer with native DBHS proteins highlights subtle conformational differences in residues at the core of the dimer interface, indicating that maximized packing of a core tryptophan residue combined with increased hydrophobic or polar complementarity at the interface is likely to determine DBHS partner preference.