Evidence for complex dynamics during U2 snRNP selection of the intron branchpoint

Area of Science:

• Molecular Biology
• RNA Splicing Mechanisms

Background:

• Pre-mRNA splicing initiates with U1 and Msl5-Mud2 binding.
• U2 binding displaces Msl5-Mud2 to form the prespliceosome, a critical step for 3' splice site choice.
• This process is linked to human cancers and myelodysplasia.

Purpose of the Study:

• To elucidate the roles of Msl5, U2, and Prp5 in prespliceosome formation.
• To understand how branch site selection influences 3' splice site choice.
• To investigate the function of cryptic branch sites (CBS) in splicing regulation.

Main Methods:

• Utilized BS-mutated ACT1 pre-mRNA to identify splicing intermediates.
• Characterized the composition and dynamics of the FIC (U2-Prp5 complex).
• Employed U2 mutants to restore U2-BS base pairing and assess splicing rescue.

Main Results:

• Msl5 remains bound to CBS in the FIC, with U2 binding downstream of the BS.
• Restoring U2-BS base pairing facilitates Prp5 and Msl5-Mud2 dissociation.
• CBS is essential for splicing rescue and FIC formation, highlighting Msl5's role in U2 recruitment.

Conclusions:

• Msl5 directs U2 to the branch site (BS).
• U2-BS base pairing is vital for releasing Prp5 and Msl5-Mud2.
• These findings offer insights into normal prespliceosome formation during pre-mRNA splicing.