spliceosome

^[1]^[2]

Function

ribonucleoprotein complex that removes introns from transcribed pre-nRNA, i.e. facilitates RNA splicing

Spliceosome assembly:

U1 snRNP binds to 5' end splice site of hnRNA & recruits other factors to form the 'E complex'
U2 snRNP is recruited to the branch region of a splice site through interactions with the 'E complex' & becomes tightly associated with the branch point sequence to form 'complex A'
a duplex is formed between U2 snRNA & the hnRNA branch region bulging out & specifying the branch adenosine for trans- esterfication
the presence of a pseudouridine residue in the U2 snRNA, nearly opposite the branch site, results in altered conformation of the RNA-RNA duplex upon U2 snRNP binding
pseudouridine places the 2'OH of the bulged adenosine in a favorable position for the 1st step of splicing
the U4/U5/U6 tri-snRNP is recruited to the assembling spliceosome to form 'complex B' & following several rearrangements, 'Complex C' (the splicesome) is activated for catalysis