Phylogeny of mRNA capping enzymes

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Eukaryotic mRNA capping (m7GpppN) is crucial for gene expression, involving RNA triphosphatase, guanylyltransferase, and methyltransferase activities.
• Enzyme complexity varies: vaccinia virus has a multifunctional polypeptide, while fungi and Chlorella virus have monofunctional guanylyltransferases.
• Transguanylylation proceeds via a covalent enzyme-GMP intermediate, with conserved active site motifs in yeast and viral capping enzymes.

Purpose of the Study:

• To conduct a structure-function analysis of the six conserved motifs in Saccharomyces cerevisiae guanylyltransferase (Ceg1).
• To elucidate the structural basis of covalent catalysis in mRNA capping enzymes.
• To identify and characterize the capping enzyme in Caenorhabditis elegans.

Main Methods:

• Targeted mutagenesis of Ceg1 to identify essential amino acid functional groups (acidic, basic, aromatic).
• Comparative analysis of mutational data with the crystal structure of Chlorella virus capping enzyme.
• Bioinformatic analysis and domain comparison to identify the Caenorhabditis elegans capping enzyme.

Main Results:

• Essential acidic, basic, and aromatic residues within Ceg1's active site motifs were identified.
• Structural insights into covalent catalysis were gained by correlating Ceg1 mutations with viral enzyme structures.
• The Caenorhabditis elegans capping enzyme was identified, featuring a conserved C-terminal guanylyltransferase domain and a novel N-terminal protein phosphatase domain.

Conclusions:

• The study clarifies the structure-function relationships of mRNA capping enzymes, particularly guanylyltransferases.
• The identification of the Caenorhabditis elegans capping enzyme expands our understanding of mRNA capping diversity.
• The novel N-terminal phosphatase domain in the nematode enzyme suggests new roles in RNA processing or regulation.