Question

Explain how the spliceosome recognizes introns.

Short answer

Answer: The key steps in the splicing process include: 1) Recognition of pre-mRNA structure, such as splice sites and branch point, 2) Small Nuclear Ribonucleoproteins (snRNPs) recognition and binding to the splice sites, 3) Formation of the spliceosome complex, 4) Activation of the spliceosome through conformational changes and rearrangements, and 5) Intron removal through consecutive transesterification reactions followed by exon joining. This process results in a spliced mRNA ready for translation.

Step-by-step solution

Pre-mRNA Structure

Pre-mRNA molecules contain the coding regions (exons) and the non-coding regions (introns) that need to be removed during the splicing process. The introns have specific sequences at their ends, known as the 5' splice site, the 3' splice site, and the branch point. The consensus sequences for these sites are GU for the 5' splice site, AG for the 3' splice site, and a conserved adenine nucleotide at the branch point. The spliceosome recognizes these specific sequences to identify the introns.

Small Nuclear Ribonucleoproteins (snRNPs) Recognition

The spliceosome consists of small nuclear ribonucleoproteins (snRNPs) that recognize and bind to the specific splice sites. The major snRNPs involved in this process are U1, U2, U4, U5, and U6. Each snRNP is a complex of a small nuclear RNA (snRNA) and a protein. For example, U1 snRNA base pairs with the 5' splice site, and U2 snRNA base pairs with the branch point.

Formation of the Spliceosome

Once the snRNPs recognize the specific splice site sequences, they assemble on the pre-mRNA molecule. The U1 snRNP binds to the 5' splice site, and the U2 snRNP binds to the branch point, forming an early spliceosome complex called the A complex. U4, U5, and U6 snRNPs then join to form a fully assembled spliceosome known as the B complex.

Spliceosome Activation

To become catalytically active, the spliceosome undergoes a series of conformational changes and rearrangements. The U1 and U4 snRNPs are released, and the U6 snRNA interacts with the 5' splice site and the U2 snRNA at the branch point. This rearrangement brings the 5' and 3' splice sites and the branch point close to each other, facilitating the intron removal.

Intron Removal

The activated spliceosome then carries out two consecutive transesterification reactions. In the first reaction, the 2'-OH group of the conserved adenine at the branch point attacks the 5' splice site, resulting in the cleavage of the 5' exon-intron boundary and the formation of a lariat intron structure. In the second reaction, the 3'-OH group of the released 5' exon attacks the 3' splice site, releasing the lariat intron and joining the two exons together. The spliced mRNA, now devoid of introns and with connected exons, is ready for translation. Meanwhile, the spliceosome disassembles, and the snRNPs are recycled for further splicing events.