Spliceosomes are the large ribonucleoprotein complexes that catalyse splicing in pre-mRNA or hnRNA. They remove introns from the pre-mRNA transcript. They are found in the eukaryotic nuclei.
mRNA Synthesis and Processing
mRNA or messenger RNA is a single-stranded RNA molecule. It is complementary to the DNA and provides the template for protein synthesis. In eukaryotes, mRNA is synthesised in the nucleus from DNA by a process called transcription. RNA polymerase II catalyses the transcription of pre-mRNA.
The pre-mRNA is called hnRNA or heterogeneous nuclear RNA. The hnRNA undergoes processing to form the mature mRNA, which is then transported out of the nucleus. The translation or protein synthesis takes place in the cytoplasm.
The hnRNA undergoes post-transcriptional processing to produce mature mRNA. It contains both exons and introns, which are coding and non-coding regions respectively. The pre-mRNA undergoes splicing, capping and tailing to produce functional mRNA.
- Splicing – It is the process of removal of introns or non-coding regions from the primary transcript. Exons are joined together. The splicing is catalysed by spliceosomes.
- Capping – It is the process of the addition of cap or methyl guanosine triphosphate to the 5′-end of the primary transcript.
- Tailing – In the process, 200-300 adenylate residues are added at 3’-end. It is called poly(A) tail.
After processing of hnRNA, the fully functional mRNA is transported out of the nucleus through the nuclear pore complex and acts as a template for protein synthesis.
Now let’s learn in detail about spliceosomes.
Spliceosomes
Spliceosomes catalyse splicing of pre-mRNA transcript. Splicing is one of the steps in post-transcriptional processing, which is necessary for the synthesis of functional mRNA in eukaryotes. Splicing occurs in the nucleus. In this process, the intervening sequences or introns of pre-mRNA that are non-coding, are removed, and coding sequences or exons are joined together.
Sometimes the RNA itself catalyses its own splicing. It is called the ribozyme. Spliceosomes are not required in self-splicing.
Spliceosome is a huge complex made up of small nuclear ribonucleoproteins (snRNPs or called ‘snurps’). The spliceosome complexes are multi-megadalton in size. In humans, around 300 different types of proteins associate, to form a spliceosome complex. There are around 100,000 spliceosomes present in each cell that remove various intron sequences.
Some of the proteins of spliceosomes are specific to RNA and some drive the process to ensure the accuracy of the process. The small nuclear RNAs (snRNAs) bind to other proteins to form small nuclear riboprotein complexes (snRNPs). The snRNAs that make up the spliceosome complex, are rich in uridine. They are called U1, U2, U4, U5, U6, etc. There are two types of spliceosomes:
- Major Spliceosomes – They account for the removal of 99.5% of introns. They are formed from U1, U2, U4, U5 and U6 snRNPs.
- Minor Spliceosomes – They account for the removal of 0.5% of introns. They are formed from U11, U12, U4atac, U5, and U6atac snRNPs.
Mechanism of Splicing
Splicing is the process of removal of introns and ligation of exons in the pre-mRNA or hnRNA.
Spliceosome assembly occurs at the exon-intron junction in hnRNA. The U1 snRNP recognises the beginning of an intron (5’-end). The U2 snRNP recognises the specific site near the 3’-end of an intron. The other components assemble and rearrangement occurs.
The splicing occurs in two steps:
- The first step is the cleavage at the 5’ splice site of the intron.
- The second step is the linking of exons and the cleavage at the 3’ splice site of the intron. Both of these reactions occur together.
Spliceosomes then disassemble from the removed intron and the intron is degraded.
Most genes in humans contain introns, therefore spliceosomes affect gene expression. Some genes show alternative splicing, wherein a single gene produces multiple RNAs that code for different proteins.
Incorrect splicing of a gene or misregulation of a spliceosome may lead to mutation and genetic diseases. A single point mutation can lead to a considerable change in protein structure and its abundance.
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