Single-Molecule Analysis of Pre-mRNA Splicing with Colocalization Single-Molecule Spectroscopy (CoSMoS)

Comparative Colocalization Single-Molecule Spectroscopy (CoSMoS) with Multiple RNA Species

Colocalization single-molecule spectroscopy (CoSMoS) allows studying RNA-protein complexes in the full complexity of their cellular environment at single-molecule resolution. Conventionally, the interaction between a single RNA species and multiple proteins is monitored in real time. However, comparing interactions of the same proteins with different RNA species in the same cell extract promises unique insights into RNA biology. Here, we describe an approach to monitor multiple RNA species simultaneously to enable direct comparison. This approach represents a technological development to avoid conventional inter-experiment comparisons.

Preparation of SNAPf-Beads for Colocalization Single-Molecule Spectroscopy (CoSMoS) of RNA-Protein Complexes

The SNAP_f-tag is a chemical tag that allows rapid and highly specific covalent labeling of proteins even in the full complexity of the cellular environment. The SNAP_f-tag has been instrumental to study native RNA-protein complexes at single-molecule resolution in their cellular environment as efficient labeling of the RNAs and proteins of interest is essential for this colocalization single-molecule spectroscopy (CoSMoS) technique. However, removal of excessive benzylguanine dye after the labeling reaction has remained challenging. Here, we describe a strategy to remove excessive benzylguanine dye using SNAP_f-tag coated beads as sponges.

Synergistic assembly of human pre-spliceosomes across introns and exons

Most human genes contain multiple introns, necessitating mechanisms to effectively define exons and ensure their proper connection by spliceosomes. Human spliceosome assembly involves both cross-intron and cross-exon interactions, but how these work together is unclear. We examined in human nuclear extracts dynamic interactions of single pre-mRNA molecules with individual fluorescently tagged spliceosomal subcomplexes to investigate how cross-intron and cross-exon processes jointly promote pre-spliceosome assembly. U1 subcomplex bound to the 5' splice site of an intron acts jointly with U1 bound to the 5' splice site of the next intron to dramatically increase the rate and efficiency by which U2 subcomplex is recruited to the branch site/3' splice site of the upstream intron. The flanking 5' splice sites have greater than additive effects implying distinct mechanisms facilitating U2 recruitment. This synergy of 5' splice sites across introns and exons is likely important in promoting correct and efficient splicing of multi-intron pre-mRNAs.

A gene is a segment of DNA that usually carries the information required to build a protein, the molecules responsible for most of life’s processes. This DNA segment is organized in modules, with coding sections separated by portions of non-coding DNA known as introns.

When a gene is ‘turned on’, it gets faithfully copied into a molecule of pre-messenger RNA (pre-mRNA), which contains the alternating coding and non-coding modules. Before it can serve as a template to create a protein, this pre-mRNA must be processed and all the introns removed by a structure called the spliceosome. If this delicate process goes wrong, inaccurate protein templates are produced that may be damaging for the cell.

Spliceosomes are precise molecular ‘scissors’ that can recognize where a coding module stops and an intron starts, and then make a snip in the pre-mRNA to remove the non-coding sequence. The spliceosome is a complex molecular machine formed of numerous parts – including one known as U1 snRNP – that must come together. When a pre-mRNA has several introns, a spliceosome assembles anew for each of them.

Braun et al. designed a new method that allows them to ‘tag’ spliceosomes extracted from a human cell and follow them as they come together. The experiments show that spliceosomes working on different introns in the same pre-mRNA actually help each other out. As one assembles, this helps the spliceosome that processes the neighboring intron to get built. In particular, the U1 snRNPs processing nearby introns collaborate to promote the assembly and activity of the spliceosomes. This teamwork is likely important to guarantee that multiple introns are cut out quickly and accurately.