Tuesday, 5 May 2015

Molecule of the Month: May 2015 SFPQ

This month's choice of molecule is a little different than my April 1st spoof. It is a protein that first came across my radar over 10 years ago and is a protein with an unusual structure but with as yet an unclear Biological function. I was working closely with a long-standing collaborator and friend Dr. Doug Gjerde (CEO of Phynexus), trying to optimise conditions for the purification of protein complexes, using a novel device which Doug and his team in San Jose had developed (the Phynexus open tube capillaries are shown left). In short, we were trying to demonstrate the capture of His tagged recombinant proteins from complex mixtures and I chose HeLa cell nuclei as a proof of concept sample. We spiked the extracts with a range of recombinant His-tagged proteins and, to our surprise we obtained a highly purified protein fraction consistently contaminated with a pair of proteins, which we established, with the help of Dr. Mark Dickman's mass spectrometry expertise, belonged to a large protein assembly with two proteins PSF (polypyrimidine tract-binding protein-associated splicing factor) and p54nrbNonO (a related splicing-transcription factor) at the core. My interest  intensified when my PhD students at the time, John Ashby and Alex Bloom carried out further experiments; one of which showed the purified protein to form relatively large particles under the electron microscope. We published the methodology and our original data, but the challenge of isolating these complexes for high resolution structural studies proved fruitless in my lab.

Roll on around 10 years, and a visitor from Perth Australia, Charlie Bond, who had just begun publishing some interesting crystallography on this class of proteins, arrived in Sheffield. Charlie had managed to express recombinant derivatives of  parts of the molecular assembly (see RHS) and revealed the unusually long alpha helical element that lies at the heart of this unusual polypeptide's ability to form supramolecular assemblies. This V-shaped molecule has two unusually extended alpha helices, which stabilise each other and comprise an eleven hydrophobic amino acid periodic repeat (normally such repeats are 7 amino acids, in for example Leucine Zippers). This anti-parallel helical arrangement sits astride protein:protein and RNA binding elements, both of which are central to the biological function of this class of molecules.

Figure 1.In the April edition of Nucleic Acids Research, Charlie and his collaborators have published the crystal structure of the related molecule, which we refer to as PSF (above), but which has now been renamed as SFPQ, referring to Splicing Factor Proline and Glutamine Rich (remember your one-letter code!). The protein contains a very similar ant-parallel coiled coil structure, which provides the key element in enabling these proteins to form infinite length, extended polymers, with an intrinsic level of curvature. Hence, they may form the structural basis of the "large" circular objects we (and others) have observed when these classes of molecules are purified under certain conditions. Taken together the emerging Biological properties associated with or facilitated by molecules like SFPQ, look like they will shed new light on the complex structure function and scaffolding phenomena observed in our nuclei. Charlie's team have paved the way from what I believe will be some exciting molecular cell biology over the next few years!

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