Function of Ceg1p and the Cet1-Ceg1 mRNA Capping Complex
Ceg1p is an mRNA guanylyltransferase in Saccharomyces cerevisiae that forms a heterotetramer with the RNA triphosphatase Cet1p which is called the mRNA capping apparatus. This complex is recruited by RNA polymerase II (RNAP-II) and performs the first two steps in the 5’-guanisine mRNA capping mechanism. Cet1p hydrolyzes a phosphate from the 5’-triphosphate end of the mRNA.[1] The Ceg1p guanylyltransferase then adds a guanosine monophosphate to the 5' end of the mRNA, creating a 5'-5' triphosphate linkage. In the final step, an RNA methyltransferase comes and adds a methyl group to the N7 atom of the guanine, completing the 5'-guanine cap.[2] This 5’ capping is necessary for cells to live in yeast among other organisms. The interactions between Cet1p and Ceg1p are extremely important for inducing capping activity. These interactions both stimulate Ceg1p GMP transfer and help localize the complex in the nucleus where RNAP-II can recruit it.[3] The capping apparatus is specially recruited to the Rpb1p subunit of RNAP-II. It binds to the serine-5-phosphorylated-carboxy terminal domain (CTD) of Rpb1p. The capping mechanism occurs while transcription is still occurring, beginning when around 20-25 nucleotides are successfully transcribed by RNAP-II.[2] Another role that the Cet1-Ceg1 complex has is the suppression of RNAP-II transcription. This transcriptional control involves the capping complex keeping RNAP-II from reinitiating transcription.[4] The capping complex has also been suggested to have regulatory roles in other cellular functions such as cell proliferation and RNA interference.[2]
Structure of Ceg1p and Interactions with Cet1p in the Capping Apparatus
The is composed of a and two , one on each end of the homodimer. The capping apparatus may also have only one Ceg1p monomer attached, forming a heterotrimer complex. A contains two major domains: a nucleotydil transferase (NT) domain and an oligonucleotide binding (OB) domain. The OB domain interacts with Cet1p, while the NT domain interacts with RNAP-II. For the Cet1p, only amino acids 241-549 of each monomer are required for the complex to properly cap mRNA.[1] The Ceg1p monomers interact with the Cet1p homodimer though the of Cet1p
Specific Binding Activity of Ceg1p and the Cet1-Ceg1 Capping Apparatus
About This Structure
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