Regulation of mRNA-5'cap-related posttranscriptional gene expression events by Dcs2 a novel modulatory protein

The information contained in the genes of living cells has to be converted into cellular components that form structures and enable biochemical reactions to take place. This process is called gene expression and it is vital to all life. Gene expression comprises two main steps, called transcription and translation. In transcription, the information in the DNA sequences of the genes is converted into equivalent sequences in so-called messenger RNA (mRNA) molecules. In translation, the mRNA molecules are 'read' by a large molecular structure called the ribosome, which uses the information to dictate the synthesis of proteins. Each cell has to synthesize and break down its mRNA molecules, because this continuous turnover allows the cell to adapt its patterns of gene expression more rapidly to changes in the demand for particular proteins which are needed under certain growth conditions. This grant proposal describes how we wish to understand this metabolism of mRNA better by studying a protein (called Dcs2) that regulates a component step in the metabolism of RNA. More specifically, cells of animals (including man), plants, insects and fungi modify the beginning (called the 5'end) of each mRNA molecule by adding a methylated guanosine base in a structure called the cap. We have discovered that the regulatory protein under study determines how rapidly this cap structure is removed. This process of removing the cap structure ('decapping') affects the levels of RNA decay intermediates in the cell that are capable of inhibiting important cellular processes. Moreover, we have preliminary evidence that the availability of the methylated G base found in the cap structure influences the number of errors accumulating in cellular nucleic acids, and thereby the rate of cellular ageing. For these, and other, reasons, we wish to understand the mode of action and functional significance of the newly discovered regulatory protein Dcs2.

The m7G cap at the 5'end of mRNA plays an important role in many eukaryotic processes, including translation, polyadenylation, splicing, mRNA export and mRNA decay. The removal of the 5'cap (decapping) is therefore also of potential significance to all of these processes. The 'scavenger'-type pyrophosphatase in S.cerevisiae, Dcs1, decaps short capped RNA species (length less than 10 nucleotides most efficiently) as well as m7GDP, the product of Dcp1/Dcp2-catalysed decapping of longer mRNAs. This overall group of short m7G-oligoribonucleotides is known to exert inhibitory effects on a number of the processes mentioned above. Moreover, we have obtained evidence that m7G-misincorporation into nucleic acids can lead to accelerated cell senescence. We have discovered that a close paralogue of Dcs1 in yeast, Dcs2, is a new type of stress-induced regulator of scavenger decapping activity. Dcs2 acts by forming a heterodimer together with Dcs1, thus modifying the latter's catalytic behaviour. Preliminary data indicate that Dcs2 co-localizes with other members of the yeast mRNA decay pathway in P bodies. The proposed project will investigate in detail the role of this new regulator in controlling m7GMP-generating decapping, m7G-misincorporation into nucleic acids and ageing, the interactions between Dcs2 and other yeast proteins (including the components of the P bodies), and will also elucidate the regulatory mechanisms underlying stress-induced DCS2 expression.