Reviewer #1 (Public Review):
In this manuscript, Hansen and coworkers make use of the powerful, single-molecule assay CoSMoS to study the recognition of the 5' splice site by the U1 snRNP. Specifically, they investigate how 5' splice site oligos interacts with purified U1 snRNP to isolate 5' splice site-binding from other factors, including the CBC, BBP, and any other factors in whole cell extract that may impact binding; previous studies have investigated binding in vivo or in cellular extracts or with limited quantitative capabilities. The authors find evidence for a reversible, two-step, binding reaction in which a short-lived interaction precedes a long-lived interaction and in which binding depends on the 5' splice site sequence and the 5' end of U1. The data further suggests a compelling kinetic framework for how U1 surveys nascent transcripts for a bona fide 5'SS; specifically, both authentic and inauthentic 5' splice sites form the short-lived complexes but whereas the inauthentic complex preferentially dissociates, the authentic complex preferentially proceeds to a stable complex. Using oligos with different mutations to limit base-pairing they find that at least six potential base-pairs are required for association but that a stretch of seven base-pairs, with a maximum of one mismatch, is required for the long-lived interaction, with residues near the 5' splice site playing more important roles and with length being a stronger predictor of complex lifetime than thermodynamics, with implications for splice site predictions.
The work focuses on the determinants and mechanism of the first and a pivotal step in splicing, in a manner that completes recent structural advances. The work extends findings presented in a previous publication from the lab (Larson and Hoskins, 2017) studying binding of U1 snRNP to the 5' splice site in extract. In that study, the authors provided early evidence of two-step U1 snRNP binding in the absence of the cap binding complex or the branch point binding protein, with a more stable state following a weaker state; although factors in the extract may have influenced binding, the results are not qualitatively different here. The authors also showed some evidence in the previous study that longer binding depended on crossing a threshold and did not increase further with greater stabilization. Still, this new work is of high quality with conclusions justified by the data and of significant interest to the splicing field and of general interest to those investigating binding of snRNPs to nucleic acid.
Specific Points:
1. To test and define the role of protein in the snRNP, the authors need to investigate the roles of Yhc1 and Luc7 in 5' splice site binding in this assay, particularly with respect to defining the basis of asymmetry and snRNP destasbilization.
2. The similarity or difference of the two-step recognition mechanism described here to the recognition mechanisms of other nucleic acids by other RNP complexes is unclear. The authors need to put their findings into a larger context, relating their findings to studies of analogous systems described in the literature.
3. It is important that the authors address whether they can rule out that the exclusively long-lived complexes skip the short-lived conformation.
4. Given the co-transcriptional nature of many splicing events, the authors should discuss how recruitment by RNAP II might impact the two-step process. For example, fast dissociation by short duplexes might be countered by retention of U1 locally via RNP II.