PRMT9 is a type II methyltransferase that methylates the splicing factor SAP145

Area of Science:

• Molecular Biology
• Epigenetics
• RNA Splicing

Background:

• The human genome encodes nine protein arginine methyltransferases (PRMTs) catalyzing distinct arginine methylations.
• Protein methylation plays crucial roles in various cellular processes, including RNA splicing.

Purpose of the Study:

• To identify novel binding partners and functions of PRMT9.
• To elucidate the role of PRMT9 in spliceosome assembly and function.

Main Methods:

• Co-immunoprecipitation to identify PRMT9 binding partners.
• In vitro methylation assays to characterize PRMT9 activity on SAP145.
• RNA sequencing (RNA-seq) to assess splicing changes upon PRMT9 depletion.

Main Results:

• SAP145 and SAP49 were identified as PRMT9-binding proteins, linking PRMT9 to U2 snRNP maturation.
• PRMT9 methylates SAP145 at arginine 508, producing monomethylated arginine (MMA) and symmetrically dimethylated arginine (SDMA).
• PRMT9 is the second mammalian enzyme, alongside PRMT5, capable of SDMA deposition.
• SAP145 methylation at Arg 508 creates a binding site for the SMN protein Tudor domain.
• PRMT9 depletion led to significant splicing alterations detected by RNA-seq.

Conclusions:

• PRMT9 functions as a nonhistone methyltransferase.
• PRMT9 primes the U2 snRNP for SMN interaction through SAP145 methylation.
• PRMT9 plays a critical role in regulating RNA splicing via its interaction with the SMN complex.