The beginning and end of poly(A) tails

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Proteins, which form key building blocks of our cells and are important in determining cell identity, are encoded in genes on the DNA in the nucleus of cells. Copies of part of the gene called messenger RNAs (mRNAs) are sent to the cytoplasm of the cell where they are decoded to make the proteins. Recent studies indicate that when mRNAs are made in the process called transcription, they can be imprinted with an expiry time for removal in the cytoplasm by an unknown mechanism. There is a known timer for mRNA expiration, the poly(A) tail, which runs down by removal of the A residues in the cytoplasm. It has long been thought that the initial setting of this timer (the size of the poly(A) tail) was virtually always the same and the only difference between mRNAs was in how quick A residues were removed in the cytoplasm by enzyme complexes such as the CCR4/NOT deadenylase complex.In contrast to this textbook view, we have shown that the initial setting of the poly(A) size is regulated in the nucleus during on and off switching of genes, which could explain the imprinting of an mRNA expiry time in the nucleus. We have also shown that nuclear poly(A) tail size can be regulated by the CCR4/NOT deadenylase complex. Strikingly, some very common mRNAs appear not to have the standard poly(A) tail size when they enter the cytoplasm and don't run down their poly(A) timer, indicating that their expiry is differently regulated than previously thought. Moreover, we have identified two RNA unwinding enzymes (helicases) as differential regulators of poly(A) tails on CCR4/NOT associated mRNAs. Our data suggest that how and where a poly(A) tail is generated and removed may determine its function. It shows that we don't know as much about mRNA removal as we thought we did. A better understanding of the fundamental process of poly(A) tail metabolism is essential for understanding how genes are used in healthy organisms as well as in disease.To address this question, we will use novel methods for measuring poly(A) tail sizes of thousands of mRNAs in three stages of the mRNA life cycle: as they are being made on the DNA, just before they exit the nucleus and in the cytoplasm. In one set of experiments, we will remove proteins involved in poly(A) tail regulation, including a key part of the CCR4/NOT complex and the two helicases and examine the effect on the poly(A) tail as well as on mRNA removal. By combining our data with existing data, we will be able to see in which stage each poly(A) tail regulator works and how this affects the timing of mRNA removal and the efficiency of protein synthesis. In a second set of experiments, we will look at the effects of naturally occuring poly(A) tail changes to detect at which stage their size is regulated and how this affects their stability. Our work will answer fundamental questions as well as inform current drug development programmes in this area.