Question

1. Explain the role of snRNPs in RNA processing. Be specific.

Answer 1

Answer- During mRNA processing or splicing, introns removal occurs within and is catalyzed by a large protein complex called spliceosome. The spliceisome is made up of specialized RNA protein complexes known as small nuclear ribonucleoproteins. and five RNAs i.e. U1, U2, U4, U5 and U6 and collectively called snRNAs.

The snRNPs have 3 roles in splicing

1. they recognize the 5' splice site and branch site

2. they bring those sites together as required

3. they catalyze the RNA cleavage and joining reaction.

Initially the 5' splice site is recognised by the U1snRNP.

U2AF is made up of two subunits larger one is U2AF 65 binds to Py tract and smaller is U2AF 35 binds to 3' splice site. This complex of RNA and protein is called E complex.

U2 snRNP then binds to the branch site aided by U2AF. This arrangement is called A complex. The base pairing between the branch site is such that the branch site A residue is extruded from the resulting stretch of double stranded RNA as single bulge, this A residue is thus unpaired available to react with 5' splice site.

The next step is the rearrangement of A complex to bring together all three splice sites. This is achieved with the help of U4, U6, and U5snRNPs

Now U1 leaves the complex and U6 replaces it at the 5' splice site.

Then U4 is released from the complex allowing U6 to interact with U2 this arrangement is called C complex it produces active site.

Formation of the active site juxtaposes the 5'splice site of the pre mRNA and the branch site facilitating the first transesterification reaction. The second reaction between 5'and 3' splice site is aided by U5 snRNP which help to bring the two exons together.

The final step involves release of mRNA product and the snRNPs.

GU 5 t 'spuce 3 site Branch site 3'splice st'te JUISARNA ATP grua sn RNP ADPUR AG ul U2 ATP -44-UG + US ADP + Pi UZ 31 four, UL, U4 05 31 u 6 U12 dlariat formation Lariat A intron release 3