The integrity of the U12 snRNA 3' stem-loop is necessary for its overall stability

Distinct functions for the paralogous RBM41 and U11/U12-65K proteins in the minor spliceosome

Here, we identify RBM41 as a novel unique protein component of the minor spliceosome. RBM41 has no previously recognized cellular function but has been identified as a paralog of U11/U12-65K, a known unique component of the U11/U12 di-snRNP. Both proteins use their highly similar C-terminal RRMs to bind to 3'-terminal stem-loops in U12 and U6atac snRNAs with comparable affinity. Our BioID data indicate that the unique N-terminal domain of RBM41 is necessary for its association with complexes containing DHX8, an RNA helicase, which in the major spliceosome drives the release of mature mRNA from the spliceosome. Consistently, we show that RBM41 associates with excised U12-type intron lariats, is present in the U12 mono-snRNP, and is enriched in Cajal bodies, together suggesting that RBM41 functions in the post-splicing steps of the minor spliceosome assembly/disassembly cycle. This contrasts with U11/U12-65K, which uses its N-terminal region to interact with U11 snRNP during intron recognition. Finally, while RBM41 knockout cells are viable, they show alterations in U12-type 3' splice site usage. Together, our results highlight the role of the 3'-terminal stem-loop of U12 snRNA as a dynamic binding platform for the U11/U12-65K and RBM41 proteins, which function at distinct stages of the assembly/disassembly cycle.

Introns: the "dark matter" of the eukaryotic genome

The emergence of introns was a significant evolutionary leap that is a major distinguishing feature between prokaryotic and eukaryotic genomes. While historically introns were regarded merely as the sequences that are removed to produce spliced transcripts encoding functional products, increasingly data suggests that introns play important roles in the regulation of gene expression. Here, we use an intron-centric lens to review the role of introns in eukaryotic gene expression. First, we focus on intron architecture and how it may influence mechanisms of splicing. Second, we focus on the implications of spliceosomal snRNAs and their variants on intron splicing. Finally, we discuss how the presence of introns and the need to splice them influences transcription regulation. Despite the abundance of introns in the eukaryotic genome and their emerging role regulating gene expression, a lot remains unexplored. Therefore, here we refer to introns as the "dark matter" of the eukaryotic genome and discuss some of the outstanding questions in the field.

Phylogenetic Analysis of Spliceosome SF3a2 in Different Plant Species

The formation of mature mRNA requires cutting introns and splicing exons. The occurrence of splicing involves the participation of the spliceosome. Common spliceosomes mainly include five snRNPs: U1, U2, U4/U6, and U5. SF3a2, an essential component of spliceosome U2 snRNP, participates in splicing a series of genes. There is no definition of SF3a2 in plants. The paper elaborated on SF3a2s from a series of plants through protein sequence similarity. We constructed the evolutionary relationship of SF3a2s in plants. Moreover, we analyzed the similarities and differences in gene structure, protein structure, the cis-element of the promoter, and expression pattern; we predicted their interacting proteins and constructed their collinearity. We have preliminarily analyzed SF3a2s in plants and clarified the evolutionary relationship between different species; these studies can better serve for in-depth research on the members of the spliceosome in plants.

Mutations in the non-coding RNU4ATAC gene affect the homeostasis and function of the Integrator complex

Various genetic diseases associated with microcephaly and developmental defects are due to pathogenic variants in the U4atac small nuclear RNA (snRNA), a component of the minor spliceosome essential for the removal of U12-type introns from eukaryotic mRNAs. While it has been shown that a few RNU4ATAC mutations result in impaired binding of essential protein components, the molecular defects of the vast majority of variants are still unknown. Here, we used lymphoblastoid cells derived from RNU4ATAC compound heterozygous (g.108_126del;g.111G>A) twin patients with MOPD1 phenotypes to analyze the molecular consequences of the mutations on small nuclear ribonucleoproteins (snRNPs) formation and on splicing. We found that the U4atac108_126del mutant is unstable and that the U4atac111G>A mutant as well as the minor di- and tri-snRNPs are present at reduced levels. Our results also reveal the existence of 3'-extended snRNA transcripts in patients' cells. Moreover, we show that the mutant cells have alterations in splicing of INTS7 and INTS10 minor introns, contain lower levels of the INTS7 and INTS10 proteins and display changes in the assembly of Integrator subunits. Altogether, our results show that compound heterozygous g.108_126del;g.111G>A mutations induce splicing defects and affect the homeostasis and function of the Integrator complex.

Human spliceosomal snRNA sequence variants generate variant spliceosomes

Human pre-mRNA splicing is primarily catalyzed by the major spliceosome, comprising five small nuclear ribonucleoprotein complexes, U1, U2, U4, U5, and U6 snRNPs, each of which contains the corresponding U-rich snRNA. These snRNAs are encoded by large gene families exhibiting significant sequence variation, but it remains unknown if most human snRNA genes are untranscribed pseudogenes or produce variant snRNAs with the potential to differentially influence splicing. Since gene duplication and variation are powerful mechanisms of evolutionary adaptation, we sought to address this knowledge gap by systematically profiling human U1, U2, U4, and U5 snRNA variant gene transcripts. We identified 55 transcripts that are detectably expressed in human cells, 38 of which incorporate into snRNPs and spliceosomes in 293T cells. All U1 snRNA variants are more than 1000-fold less abundant in spliceosomes than the canonical U1, whereas at least 1% of spliceosomes contain a variant of U2 or U4. In contrast, eight U5 snRNA sequence variants occupy spliceosomes at levels of 1% to 46%. Furthermore, snRNA variants display distinct expression patterns across five human cell lines and adult and fetal tissues. Different RNA degradation rates contribute to the diverse steady state levels of snRNA variants. Our findings suggest that variant spliceosomes containing noncanonical snRNAs may contribute to different tissue- and cell-type-specific alternative splicing patterns.

At the Intersection of Major and Minor Spliceosomes: Crosstalk Mechanisms and Their Impact on Gene Expression

Many eukaryotic species contain two separate molecular machineries for removing non-coding intron sequences from pre-mRNA molecules. The majority of introns (more than 99.5% in humans) are recognized and excised by the major spliceosome, which utilizes relatively poorly conserved sequence elements at the 5′ and 3′ ends of the intron that are used for intron recognition and in subsequent catalysis. In contrast, the minor spliceosome targets a rare group of introns (approximately 0.5% in humans) with highly conserved sequences at the 5′ and 3′ ends of the intron. Minor introns coexist in the same genes with major introns and while the two intron types are spliced by separate spliceosomes, the two splicing machineries can interact with one another to shape mRNA processing events in genes containing minor introns. Here, we review known cooperative and competitive interactions between the two spliceosomes and discuss the mechanistic basis of the spliceosome crosstalk, its regulatory significance, and impact on spliceosome diseases.