A SERINE ARGININE-RICH NUCLEAR MATRIX CYCLOPHILIN INTERACTS WITH THE C-TERMINAL DOMAIN OF RNA-POLYMERASE-II/

The largest subunit of RNA polymerase II shows a striking difference i n the degree of phosphorylation, depending on its functional state: in itiating and elongating polymerases are unphosphorylated and highly ph osphorylated respectively. Phosphorylation mostly occurs at the C-term inal domain (CTD), which consists of a repetitive heptapeptide structu re. Using the yeast two-hybrid system, we have selected for mammalian proteins that interact with the phosphorylated CTD of mammalian RNA po lymerase II. A prominent isolate, designated SRcyp/CASP10, specificall y interacts with the CTD not only in vivo but also in vitro. It contai ns a serine/arginine-rich (SR) domain, similar to that found in the SR protein family of pre-mRNA splicing factors, which is required for in teraction with the CTD. Most remarkably, the N-terminal region of SRcy p includes a peptidyl-prolyl cis-trans isomerase domain characteristic of immunophilins/cyclophilins (Cyp), a protein family implicated in p rotein folding, assembly and transport. SRcyp is a nuclear protein wit h a characteristic distribution in large irregularly shaped nuclear sp eckles and co-localizes perfectly with the SR domain-containing splici ng factor SC35. Recent independent investigations have provided comple mentary data, such as an association of the phosphorylated form of RNA polymerase II with the nuclear speckles, impaired splicing ina CTD de letion background and inhibition of in vitro splicing by CTD peptides. Taken together, these data indicate that factors directly or indirect ly involved in splicing are associated with the elongating RNA polymer ases, from where they might translocate to the nascent transcripts to ensure efficient splicing, concomitant with transcription.