Spliceosome
The spliceosome or spliceosome is a structure in the eukaryotic cell nucleus that participates in gene expression . It catalyzes splicing , removing the introns (non-coding sections) from the pre-mRNA and linking the exons (coding sections). The spliceosome is therefore involved in the maturation of the pre-mRNA into mRNA.
structure
The spliceosome consists of 5 snRNPs (small nuclear ribonucleoprotein particles), each of which contains a snRNA (small nuclear RNA) and a number of proteins . In addition, other non-snRNP factors - which are proteins or protein complexes - are involved in the structure. Overall, the mass of the assembled spliceosome is over 13 megadaltons .
to form
A distinction is made between the major spliceosome (from the U1, U2, U4, U5 and U6 snRNPs ), which processes more than 95% of human introns, and the minor spliceosome, which mainly processes the so-called ATAC introns and also has a different snRNP composition ( U11 and U12 snRNP instead of U1 and U2 and U4atac or U6atac instead of U4 and U6) (see also splice point ).
Process of splicing
The spliceosome is assembled from its individual parts directly on the primary transcript for each reaction (i.e. the removal of an intron). The snRNP U1 ( small nuclear ribonucleoprotein U1 ) initially binds the 5 ' splice site in mammalian cells and initiates the further assembly of the spliceosome. The snRNP U2 binds the branching point of the intron with the help of other protein factors (U2AF, U2 auxilliary factor). Finally, the so-called tri-snRNP, a complex of the snRNP U4, U6 and U5, is bound. After structural rearrangements, in which other factors are involved, the active spliceosome is finally formed, which lacks the snRNP U1 and U4. snRNP U4 probably acts as an inhibitor , which is displaced by snRNP U6. The catalytic center is formed from a helix of snRNP U2 and U6. This catalyzes the first step (the first transesterification) of the splicing process, whereby the splice intermediates (free 5'-exon and 3'-exon intronlariat) arise. After further changes to the structure and composition of the spliceosome, the second reaction step finally takes place, which finally leads to the release of the products and the disintegration of the spliceosome.
literature
- Choice, MC. et al . (2009): The spliceosome: design principles of a dynamic RNP machine . In: Cell 136 (4); 701-718; PMID 19239890 ; doi : 10.1016 / j.cell.2009.02.009
- Staley, JP. and Woolford, JL. Jr. (2009): Assembly of ribosomes and spliceosomes: complex ribonucleoprotein machines . In: Curr Opin Cell Biol . 21 (1); 109-118; PMID 19167202 ; doi : 10.1016 / j.ceb.2009.01.003
- Reinhard Lührmann : Structure and Function of Spliceosomes, Research Report 2012 MPI for Biophysical Chemistry, Yearbook 2012 of the Max Planck Society, online
