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The CrkRS/CDK12 kinase is activated by a novel isoform of Cyclin K and phosphorylates the C‐terminal domain of RNA Pol II
Author(s) -
Morin Gregg B,
Kuzyk Michael A,
Tien Jerry F,
Schaeffer Emily,
Cheng S.W. Grace
Publication year - 2009
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.23.1_supplement.492.3
Subject(s) - cyclin d , cyclin dependent kinase complex , cyclin a , cyclin a2 , cyclin dependent kinase , cyclin , microbiology and biotechnology , cyclin dependent kinase 9 , cyclin b , cyclin e , biology , chemistry , cyclin dependent kinase 2 , kinase , biochemistry , cell cycle , protein kinase a , cell
CrkRS (Cdc2‐related kinase, Arg/Ser), or CDK12 (Cyclin Dependent Kinase 12), is a ser/thr kinase believed to coordinate transcription and RNA splicing. While CrkRS complexes were known to phosphorylate the C‐Terminal Domain (CTD) of RNA Pol II, the responsible cyclin was not known. However, others showed that co‐expression of CrkRS and Cyclin L could affect the alternative splicing of a model substrate. Using immunoprecipitation and mass spectrometry (IP‐MS), we identified the protein constituents of endogenous CrkRS complexes. In addition to several spliceosome‐related proteins, we identified a novel 65 kDa isoform of Cyclin K, whereas the known isoform is 43 kDa. We did not observe Cyclin L. Previously, endogenous Cyclin K complexes were known to phosphorylate the CTD, yet the responsible kinase was not identified. Using yeast‐two hybrid and co‐expression experiments others identified CDK9 as a Cyclin K partner and that CDK9/Cyclin K could regulate transcription. Using IP‐MS, we show that Cyclin K complexes isolated from mammalian cells contain CrkRS but do not contain CDK9, and that CDK9 complexes contain Cyclin T1 and not Cyclin K, within detection limits. The co‐expression of CrkRS with either isoform of Cyclin K showed increased CTD kinase activity compared to controls. Our data suggest the primary cyclin partner for CrkRS is Cyclin K and that Cyclin K is sufficient to activate CrkRS to phosphorylate the CTD.