Identification of Transient Intermediates During Spliceosome Activation by Single Molecule Fluorescence Microscopy

Spliceosome activation is the process of creating the catalytic site for RNA splicing and occurs de novo on each intron following spliceosome assembly. Dozens of factors bind to or are released from the activating spliceosome including the Lsm2-8 heteroheptameric ring that binds the U6 small nuclear RNA (snRNA) 3’-end. Lsm2-8 must be released to permit active site stabilization by the Prp19-containing complex (NineTeen Complex, NTC); however, little is known about the temporal order of events and dynamic interactions that lead up to and follow Lsm2-8 release. We have used colocalization single molecule spectroscopy (CoSMoS) to visualize Lsm2-8 dynamics during activation of yeast spliceosomes. Lsm2-8 is recruited as a component of the tri-snRNP and is released after integration of the Prp19-containing complex (NineTeen Complex, NTC). Despite Lsm2-8 and the NTC being mutually exclusive in existing cryo-EM structures of yeast B complex spliceosomes, we identify a transient intermediate containing both ![Graphic][1] and provide a kinetic framework for its formation and transformation during activation. Prior to ![Graphic][2] assembly, the NTC rapidly and reversibly samples the spliceosome suggesting a mechanism for preventing NTC sequestration by defective spliceosomes that fail to properly activate. In complementary ensemble assays, we show that a base-pairing dependent ternary complex can form between Lsm2-8 and U2 and U6 helix II RNAs. Together our data suggest a Hfq-like function for Lsm2-8 in maintaining U2/U6 helix II integrity before it can be transferred to the NTC by transient formation of the ![Graphic][3] spliceosome. Significance Statement The spliceosome active site is created de novo during activation and involves numerous conformational and compositional changes. Here, we define a kinetic pathway for yeast spliceosome activation using single molecule fluorescence that includes transient intermediates not previously identified. Real-time measurements allow us to uncover rapid, reversible sampling interactions of the NineTeen Complex (NTC) that may prevent its accumulation on defective spliceosomes. By analogy with bacterial Hfq, we propose that the homologous Lsm2-8 proteins stabilize U2/U6 helix II during activation before the helix is transferred to the NTC in a short-lived spliceosome containing both Lsm2-8 and the NTC. Our data demonstrate how single molecule studies of activation can reveal kinetically-competent intermediates and complement cryo-EM studies of stalled or inhibited complexes. ### Competing Interest Statement AAH is carrying out sponsored research for Remix Therapeutics and is a member of their scientific advisory board. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif