Ceg1

Ceg1 is a protein in the budding yeast Saccharomyces cerevisiae encoded by the CEG1 gene. It is a core component of the mRNA capping enzyme, which attaches the 5' cap structure to newly synthesized RNA transcripts. In yeast, the capping enzyme consists of Cet1, the RNA triphosphatase, and Ceg1, the RNA guanylyltransferase; together they form a functional complex that catalyzes cap formation by transferring GMP from GTP to the diphosphorylated 5' end of the transcript, producing the cap structure GpppN.

Ceg1 interacts with Cet1 and is recruited to RNA polymerase II transcription complexes, contributing to co-transcriptional capping. This capping step is essential for subsequent steps in RNA maturation, including methylation and proper translation, and it influences mRNA stability.

Genetically, CEG1 is essential for viable growth in Saccharomyces cerevisiae, reflecting the critical role of 5' capping in mRNA metabolism. The protein is conserved among fungi and is related to the broader eukaryotic cap formation machinery, which is organized differently across species but serves the same fundamental function of marking transcripts for processing and translation.

In terms of structure, Ceg1 contains a guanylyltransferase domain required for GMP transfer and interfaces with Cet1 and the transcriptional machinery to facilitate efficient capping. Its function as part of the mRNA capping apparatus is a key step in ensuring proper mRNA maturation and gene expression.

Overall, Ceg1 is a fundamental component of eukaryotic mRNA processing, essential for cap formation and stability, and serves as a crucial link between transcription and RNA maturation.