FBP21's C-Terminal Domain Remains Dynamic When Wrapped around the c-Sec63 Unit of Brr2 Helicase.

Based on our recent finding that FBP21 regulates human Brr2 helicase activity involved in the activation of the spliceosomal B-complex, we investigated the structural and dynamic contribution of FBP21 to the interaction. By using NMR spectroscopy, we could show that the 50 C-terminal residues of FBP21 (FBP21326-376), which are sufficient to fully form the interaction with the C-terminal Sec63 unit of Brr2 (Brr2C-Sec63), adopt a random-coil conformation in their unbound state. Upon interaction with Brr2C-Sec63, 42 residues of FBP21326-376 cover the large binding site on Brr2C-Sec63 in an extended conformation. Short charged motifs are steering complex formation, still allowing the bound state to retain dynamics. Based on fragment docking in combination with experimental restraints, we present models of the complex structure. The FBP21326-376/Brr2C-Sec63 interaction thus presents an example of an intrinsically disordered protein/ordered-protein interaction in which a large binding site provides high specificity and, in combination with conformational disorder, displays a relatively high affinity.

A new role for FBP21 as regulator of Brr2 helicase activity.

Splicing of eukaryotic pre-mRNA is carried out by the spliceosome, which assembles stepwise on each splicing substrate. This requires the concerted action of snRNPs and non-snRNP accessory proteins, the functions of which are often not well understood. Of special interest are B complex factors that enter the spliceosome prior to catalytic activation and may alter splicing kinetics and splice site selection. One of these proteins is FBP21, for which we identified several spliceosomal binding partners in a yeast-two-hybrid screen, among them the RNA helicase Brr2. Biochemical and biophysical analyses revealed that an intrinsically disordered region of FBP21 binds to an extended surface of the C-terminal Sec63 unit of Brr2. Additional contacts in the C-terminal helicase cassette are required for allosteric inhibition of Brr2 helicase activity. Furthermore, the direct interaction between FBP21 and the U4/U6 di-snRNA was found to reduce the pool of unwound U4/U6 di-snRNA. Our results suggest FBP21 as a novel key player in the regulation of Brr2.

Intrinsically Disordered Protein Ntr2 Modulates the Spliceosomal RNA Helicase Brr2.

Precursor messenger RNA splicing is mediated by the spliceosome, a large and dynamic molecular machine composed of five small nuclear RNAs and numerous proteins. Many spliceosomal proteins are predicted to be intrinsically disordered or contain large disordered regions, but experimental validation of these predictions is scarce, and the precise functions of these proteins are often unclear. Here, we show via circular dichroism spectroscopy, dynamic light scattering, and NMR spectroscopy that the yeast spliceosomal disassembly factor Ntr2 is largely intrinsically disordered. Peptide SPOT analyses, analytical size-exclusion chromatography, and surface plasmon resonance measurements revealed that Ntr2 uses an N-terminal region to bind the C-terminal helicase unit of the Brr2 RNA helicase, an enzyme involved in spliceosome activation and implicated in splicing catalysis and spliceosome disassembly. NMR analyses suggested that Ntr2 does not adopt a tertiary structure and likely remains disordered upon complex formation. RNA binding and unwinding studies showed that Ntr2 downregulates Brr2 helicase activity in vitro by modulating the fraction of helicase molecules productively bound to the RNA substrate. Our data clarify the nature of a physical link between Brr2 and Ntr2, and point to the possibility of a functional Ntr2-Brr2 interplay during splicing.

Direct identification of ligand-receptor interactions on living cells and tissues.

Many cellular responses are triggered by proteins, drugs or pathogens binding to cell-surface receptors, but it can be challenging to identify which receptors are bound by a given ligand. Here we describe TRICEPS, a chemoproteomic reagent with three moieties--one that binds ligands containing an amino group, a second that binds glycosylated receptors on living cells and a biotin tag for purifying the receptor peptides for identification by quantitative mass spectrometry. We validated this ligand-based, receptor-capture (LRC) technology using insulin, transferrin, apelin, epidermal growth factor, the therapeutic antibody trastuzumab and two DARPins targeting ErbB2. In some cases, we could also determine the approximate ligand-binding sites on the receptors. Using TRICEPS to label intact mature vaccinia viruses, we identified the cell surface proteins AXL, M6PR, DAG1, CSPG4 and CDH13 as binding factors on human cells. This technology enables the identification of receptors for many types of ligands under near-physiological conditions and without the need for genetic manipulations.