Sandbox SRp20 John Davis

SRp20 is one of the smallest members of Ser- and Arg-rich protein family with 150 amino acids in the protein. The RNA recognition motif (RRM)of SRp20 has a βαββαβ topology, with two α-helices packed against one side of the four-stranded . The β-sheet surface has a large hydrophobic core with the aromatic amino acids Tyr, Phe, and Trp. The aromatic amino acid residues in the β-sheet are what cause the affinity of RNA for SRp20. When RNA binds to to SRp20, 3-8 nucleotides (C,U,A) in the RNA bind to the four-stranded β-sheet in the . TAP binds to the α-helices that are opposite the β-sheet in the RRM. So, RNA binds to one side of the RRM and TAP binds to the other.(1)

Function

SRp20 is important for alternative RNA splicing. SRp20 can work with the C-terminal domain (CTD) of RNA pol II to remove exons from RNA after transcription. A specific mechanism for the interaction between the CTD and SRp20 is unknown. However, it is clear that the CTD coordinates the activity of SRp20 and SRp20 has been shown to preferentially associate with sites of RNA pol II transcription. (2) This demonstrates that SRp20 binds specific introns for RNA pol II to cut.

SRp20 is also important for nucleocytoplasmic export of mRNA. Tip-associated protein (TAP) is an export factor for that causes export of mRNA from the nucleus to the cytoplasm.(1) SRp20 functions as an adaptor protein that binds directly to a section of the properly-spliced, 22-nucleotide element of mRNA.(3) Once mRNA is bound to SRp20, TAP binds to the Arg-rich α-helices on the RRM of SRp20. SRp20 brings TAP and mRNA into close contact with each other, allowing for mRNA to bind to TAP and exit the nucleus. (4)