Cytoplasmic Polyadenylation Promotes Translation of Some mRNAs

In addition to miRNAs, several protein-mediated translational controls help regulate the expression of some genes. Regulatory elements in mRNAs that interact with specific proteins to control translation are generally present in the UTR at the 3′ or 5′ end of an mRNA. Here we discuss a type of protein-mediated translational control involving 3′ regulatory elements. A different mechanism, involving RNA-binding proteins that interact with 5′ regulatory elements, is discussed later.

Translation of many eukaryotic mRNAs is regulated by sequence-specific RNA-binding proteins that bind cooperatively to neighboring sites in 3′ UTRs. This allows them to function in a combinatorial manner similar to the cooperative binding of transcription factors to regulatory sites in an enhancer or promoter region. In most cases studied, translation is repressed by protein binding to 3′ regulatory elements, and regulation results from derepression at the appropriate time or place in a cell or developing embryo. The mechanism of such repression is best understood for mRNAs that must undergo cytoplasmic polyadenylation before they can be translated.

Cytoplasmic polyadenylation is a critical aspect of gene expression in the early embryos of animals. The egg cells (oocytes) of multicellular animals contain many mRNAs, encoding numerous different proteins, that are not translated until after the egg is fertilized by a sperm cell. Some of these “stored” mRNAs have a short poly(A) tail, consisting of only 20–40 A residues, to which only a few molecules of cytoplasmic poly(A)-binding protein (PABPC1) can bind. As discussed in Chapter 5, multiple PABPC1 molecules bound to the long poly(A) tail of an mRNA interact with the eIF4G initiation factor, thereby stabilizing the interaction of the mRNA 5′ cap with eIF4E, which is required for translation initiation (see Figure 5-23). Because this stabilization cannot occur with mRNAs that have short poly(A) tails, such mRNAs are not translated efficiently. At the appropriate time during oocyte maturation or after fertilization, usually in response to an external signal, approximately 150 A residues are added to the short poly(A) tails on these mRNAs in the cytoplasm, stimulating their translation.

Studies with mRNAs stored in Xenopus oocytes have helped elucidate the mechanism of this type of translational control. Experiments in which short-tailed mRNAs were injected into oocytes have shown that two sequences in their 3′ UTRs are required for their polyadenylation in the cytoplasm: the AAUAAA polyadenylation signal that is also required for the nuclear polyadenylation of pre-mRNAs, and one or more copies of an upstream U-rich cytoplasmic polyadenylation element (CPE). This regulatory element is bound by a highly conserved CPE-binding protein (CPEB) that contains an RRM domain and a zinc-finger domain.

In the absence of a stimulatory signal, CPEB bound to the U-rich CPE interacts with the protein Maskin, which in turn binds to the eIF4E associated with the mRNA 5′ cap (Figure 10-31, left). As a result, eIF4E cannot interact with other initiation factors or the small ribosomal subunit, so translation initiation is blocked. During oocyte maturation, a specific CPEB serine is phosphorylated, causing Maskin to dissociate from the complex. This allows cytoplasmic forms of the cleavage and polyadenylation specificity factor (CPSF) and poly(A) polymerase (PAP) to bind to the mRNA cooperatively with CPEB. Once PAP catalyzes the addition of A residues, PABPC1 can bind to the lengthened poly(A) tail, leading to the stabilized interaction of all the factors needed to initiate translation (Figure 10-31, right; see also Figure 5-23). In the case of Xenopus oocyte maturation, the protein kinase that phosphorylates CPEB is activated in response to the hormone progesterone. Thus timing of the translation of stored mRNAs encoding proteins needed for oocyte maturation is regulated by this external signal.

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Considerable evidence indicates that a similar mechanism of translational control plays a role in learning and memory. In the central nervous system, the axons from a thousand or so neurons can make connections (synapses) with the dendrites of a single postsynaptic neuron (see Figure 22-31). When one of these axons is stimulated, the postsynaptic neuron “remembers” which one of these thousands of synapses was stimulated. The next time that synapse is stimulated, the strength of the response triggered in the postsynaptic cell differs from the first time. This change in response has been shown to result largely from the translational activation of mRNAs stored in the region of the synapse, leading to the local synthesis of new proteins that increase the size and alter the neurophysiological characteristics of the synapse. The finding that CPEB is present in neuronal dendrites has led to the proposal that cytoplasmic polyadenylation stimulates translation of specific mRNAs in dendrites, much as it does in oocytes. In this case, presumably, synaptic activity (rather than a hormone) is the signal that induces phosphorylation of CPEB and subsequent activation of translation.