Structural basis of the zinc-induced cytoplasmic aggregation of the RNA-binding protein SFPQ.

SFPQ is a ubiquitous nuclear RNA-binding protein implicated in many aspects of RNA biogenesis. Importantly, nuclear depletion and cytoplasmic accumulation of SFPQ has been linked to neuropathological conditions such as Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS). Here, we describe a molecular mechanism by which SFPQ is mislocalized to the cytoplasm. We report an unexpected discovery of the infinite polymerization of SFPQ that is induced by zinc binding to the protein. The crystal structure of human SFPQ in complex with zinc at 1.94 Å resolution reveals intermolecular interactions between SFPQ molecules that are mediated by zinc. As anticipated from the crystal structure, the application of zinc to primary cortical neurons induced the cytoplasmic accumulation and aggregation of SFPQ. Mutagenesis of the three zinc-coordinating histidine residues resulted in a significant reduction in the zinc-binding affinity of SFPQ in solution and the zinc-induced cytoplasmic aggregation of SFPQ in cultured neurons. Taken together, we propose that dysregulation of zinc availability and/or localization in neuronal cells may represent a mechanism for the imbalance in the nucleocytoplasmic distribution of SFPQ, which is an emerging hallmark of neurodegenerative diseases including AD and ALS.

Rapid purification method for human SFPQ by implementing zinc-induced polymerization.

Splicing factor proline- and glutamine-rich (SFPQ) is an RNA-binding protein, playing significant roles in gene regulation and subnuclear body formation. Our recent serendipitous discovery showed that SFPQ binds zinc directly and forms an infinite polymer that is induced by zinc binding to the protein. The zinc-induced reversible polymerization has led us to exploit this property to develop a rapid purification strategy for SFPQ without the use of affinity tags. In combination with the variation of ionic strength for salting-out of SFPQ, the reversible zinc-induced precipitation of SFPQ reduced the purification time required to obtain pure SFPQ to a single day. The purified protein was subjected to the previously reported crystallization condition. The resulting crystals diffracted to 2.22 Å resolution, confirming the quality of SFPQ purified with this new rapid purification strategy.

The Emerging Role of the RNA-Binding Protein SFPQ in Neuronal Function and Neurodegeneration.

RNA-binding proteins (RBPs) are a class of proteins known for their diverse roles in RNA biogenesis, from regulating transcriptional processes in the nucleus to facilitating translation in the cytoplasm. With higher demand for RNA metabolism in the nervous system, RBP misregulation has been linked to a wide range of neurological and neurodegenerative diseases. One of the emerging RBPs implicated in neuronal function and neurodegeneration is splicing factor proline- and glutamine-rich (SFPQ). SFPQ is a ubiquitous and abundant RBP that plays multiple regulatory roles in the nucleus such as paraspeckle formation, DNA damage repair, and various transcriptional regulation processes. An increasing number of studies have demonstrated the nuclear and also cytoplasmic roles of SFPQ in neurons, particularly in post-transcriptional regulation and RNA granule formation. Not surprisingly, the misregulation of SFPQ has been linked to pathological features shown by other neurodegenerative disease-associated RBPs such as aberrant RNA splicing, cytoplasmic mislocalization, and aggregation. In this review, we discuss recent findings on the roles of SFPQ with a particular focus on those in neuronal development and homeostasis as well as its implications in neurodegenerative diseases.