SnapShot: Formation of mRNPs

Multiple adaptor proteins (e.g., Yra1/Aly/REF1 and SR proteins) drive mRNP • export by loading dimeric export receptor (Mex67-Mtr2/TAP-p15/NXF1-NXT1). In yeast, TREX complex recruited to polymerase; DECD helicase Sub2 loads • Yra1 onto mRNA. In vertebrates, TREX binds cap via splicing-dependent interaction of Aly/REF1(vertebrate Yra1) with CBP80. Export coupled to transcription via SAGA chromatin-remodeling complex, • which binds TREX2 and relocates some genes to nuclear pore for export. Export coupled to 3 • ′ end processing (TREX required for 3′ end formation and release of mRNP from transcription site). Different export adaptors or receptors confer transcript specifi city, e.g., TREX • is important for export of heat-shock gene transcripts. Transit through pore via binding of Mex67/TAP/NXF1 to nucleoporin FG • repeats. mRNP remodeled on cytoplasmic side of pore by Dbp5 DEAD-box helicase • (which is activated by Gle1 and inositol phosphate). capping 7-methyl-guanosine cap added to 5 • ′ end of RNA by 3 enzymes: RNA 5′ triphosphatase (RT), guanyltransferase (GT) (bifunctional RT-GT in vertebrates), and methyltransferase (MT). Capping enzymes (RT and MT in yeast, RT/GT in vertebrates) recruited to RNA by • Ser5-phosphorylated RNA Pol II CTD. Cap added when ~20 nt of RNA is transcribed. Cap addition coupled to promoter • clearance and early transcription elongation. Nuclear cap-binding proteins (Cbc2-Sto1 and Cbp80-20) replaced by cytoplasmic • CBPs (Cdc33/eIF4E) after export. splicing Pre-mRNA splicing, a two-step trans-esterifi cation reaction • with a branched intermediate, is mediated by 5 snRNAs (in snRNPs) and 80–200 proteins. U1 snRNP, BBP/ SF1, and Mud2/U2AF recognize RNA elements to form a commitment complex. U2 snRNP joins next, followed by U5, U4/U6 snRNPs. After major rearrangements, catalytically active spliceosome contains U2,) and a GTPase (Snu114/SNRP116) facilitate rearrangements and regulate reaction fi delity. Vertebrate exon-exon junctions marked by EJC, which • triggers NMD when downstream of a stop codon. Splicing coupled to mRNP export via proteins that act in • both processes, such as Sub2/UAP56, serine-arginine rich (SR) proteins, and hnRNPs.