Structural insights into capping and decappingmRNA.

摘要

The 5' end of mRNA has a unique cap structure, m7GpppN (an N7-methyl guanosine linked by a 5'-5' triphosphate bridge to the first nucleoside of the transcript). The cap plays essential roles in the life cycle of mRNA, including two fundamental biological processes in cells, mRNA synthesis and mRNA decay. X-ray crystallography and biochemistry have been used to study the enzymetic mechanisms of mRNA cap formation and hydrolysis.; RNA triphosphatase, guanylyltransferase and guanine-N7 methyltransferase are required sequentially during the initial stage of mRNA transcription elongation to catalyze the formation of mRNA caps. A triphosphatase and guanylyltransferase complex (Cet1-Ceg1) from Saccharomyces cerevisiae represents a type of capping complex that is only utilized by protozoa, eukaryotic viruses and fungi. Our crystallographic work has revealed that the Cet1-Ceg1 complex is a tetramer, composed of 2 Cet1 and 2 Ceg1 molecules. The crystal structure of the Cet1-Ceg1 complex will be a foundation that will lead to the discovery of anti-protozoal, anti-viral and anti-fungal drugs, and will open a view to co-transcriptional mRNA processing. As the formation of the mRNA cap is a critical step for mRNA synthesis and maturation, the hydrolysis of the mRNA cap is an indispensable step for mRNA decay. Dcp1-associated Dcp2, an enzyme containing a Nudix (nucleoside diphosphate linked moiety X&barbelow;) domain, binds mRNA body and hydrolyzes the mRNA cap, releasing m7GDP Cap hydrolysis by Dcp2 exposes the 5' end of mRNA to 5'-3' exoribonuclease activities. Alternatively, the scavenger decapping enzyme, DcpS, exclusively binds m7GpppN and catalyzes a metal-independent decapping reaction that releases m7GMP, only after the exosome degrades mRNA body from the 3' end. DcpS is a member of the HIT (histidine t&barbelow;riad) protein family. The first crystal structure of DcpS from human in complex with cap analogs has revealed an asymmetric homodimer with two domains. The structural analysis has indicated that a dramatic conformational change is required for catalysis, and the substrate-length specificity is carried out by applying either a steric or an entropic mechanism.