Search

RNA polymerase II transcription is associated with cyclic phosphorylation of the C-terminal domain (CTD) of the large subunit of RNA polymerase II. To date, FCP1 is the only specific CTD phosphatase, which is required for general transcription and cell viability. To identify FCP1-associated proteins, we constructed a human cell line expressing epitope-tagged FCP1. In addition to RAP74, a previously identified FCP1 interacting factor, we determined that FCP1-affinity purified extracts contain RNAPII that has either a hyper- or a hypo-phosphorylated CTD. In addition, by mass spectrometry of affinity purified FCP1-associated factors, we identified a novel FCP1 -interacting protein, named MEP50, a recently described component of the methylosome complex that binds to the snRNP's Sm proteins. We found that FCP1 specifically interacts with components of the spliceosomal U small nuclear ribonucleoproteins. These results suggest a putative role of FCP1 CTD-phosphatase in linking the transcription elongation with the splicing process.

Selenoprotein synthesis requires the co-translational recoding of a UGA Sec codon. This process involves an RNA structural element, called Selenocysteine Insertion Sequence (SECIS) and the SECIS binding protein 2 (SBP2). Several selenopro-tein mRNAs undergo unusual cap hypermethylation by the trimethylguanosine synthase 1 (Tgs1), which is recruited by the ubiquitous Survival of MotoNeu-rons (SMN) protein. SMN, the protein involved in spinal muscular atrophy, is part of a chaperone complex that collaborates with the methylosome for RNP assembly. Here, we analyze the role of individual SMN and methylosome components in selenoprotein mRNP assembly and translation. We show that SBP2 interacts directly with four proteins of the SMN complex and the methylosome core proteins. Nevertheless, SBP2 is not a methylation substrate of the methylosome. We found that both SMN and methylosome complexes are required for efficient translation of the selenoprotein GPx1 in vivo. We establish that the steady-state level of several selenoprotein mRNAs, major regulators of oxidative stress damage in neurons, is specifically reduced in the spinal cord of SMN-deficient mice and that cap hypermethylation of GPx1 mRNA is affected. Altogether we identified a new function of the SMN complex and the methylosome in selenoprotein mRNP assembly and expression.