An Essential Cytoplasmic Function for the Nuclear Poly(A) Binding Protein, PABP2, in Poly(A) Tail Length Control and Early Development in Drosophila

Abstract

Translational control of maternal mRNA through regulation of poly(A) tail length is crucial during early development. The nuclear poly(A) binding protein, PABP2, was identified biochemically from its role in nuclear polyadenylation. Here, we analyze the in vivo function of PABP2 in Drosophila. PABP2 is required in vivo for polyadenylation, and Pabp2 function, including poly(A) polymerase stimulation, is essential for viability. We also demonstrate an unanticipated cytoplasmic function for PABP2 during early development. In contrast to its role in nuclear polyadenylation, cytoplasmic PABP2 acts to shorten the poly(A) tails of specific mRNAs. PABP2, together with the deadenylase CCR4, regulates the poly(A) tails of oskar and cyclin B mRNAs, both of which are also controlled by cytoplasmic polyadenylation. Both Cyclin B protein levels and embryonic development depend upon this regulation. These results identify a regulator of maternal mRNA poly(A) tail length and highlight the importance of this mode of translational control.

Crossing the borders: Poly(A)-binding proteins working on both sides of the fence

“…38 Interestingly, mutations in the gene encoding the Drosophila PABPN1 ortholog, pabp2, cause poly(A) tail extension for specific mRNAs regulated by cytoplasmic polyadenylation, thereby increasing the level of the proteins encoded by these transcripts. 39 Although the physical association between PABP2 and the regulated cytosolic transcripts was not demonstrated in this study, PABP2 copurifies with the cytoplasmic polyadenylation factor, Orb, thus providing support for a direct role of PABP2 in such cytoplasmic control of gene expression. Consistently, PABP2 localizes to the cytosol of oocytes and early embryos where these mRNAs are regulated.…”

“…7,29,42,43 In Drosophila, specific pabp2 mutants also show shorter poly(A) tails using total RNA or individual mRNAs. 39 Although this remains unclear, this hypoadenylation phenotype is assumed to be linked to PABP2's nuclear role since an hyperadenylation phenotype is associated with the cytosolic function of Drosophila PABP2 (discussed above). The budding yeast Saccharomyces cerevisiae has also been a valuable system to study the mechanism…”

Regulation of Cyclin A protein in meiosis and early embryogenesis

“…In Drosophila oogenesis, some mRNAs become polyadenylated at maturation, dependent on the GLD2 cytoplasmic poly(A) polymerase (2). The cyclin B mRNA is polyadenylated initially at oocyte maturation, coinciding with appearance of the protein in meiosis (3,4). Cyclin B is required for female fertility in Drosophila (5), and Cyclin B/CDK1 activity is likely to promote oocyte maturation and the release of the prophase I arrest.…”

“…Oocyte maturation in Xenopus is triggered by polyadenylation of cyclin B mRNA, leading to translation of Cyclin B and active MPF (cyclin B/CDK1) (1). In Drosophila oogenesis, poly(A) tail lengthening occurs both at oocyte maturation and activation (3). Lengthening of the poly(A) tail of cyclin B mRNA at each of these meiotic transitions is correlated with renewed appearance of the protein (3,4), and polyadenylation of the mRNA for the meiosis-specific APC/C activator Cortex by the GLD2 cytoplasmic poly(A) polymerase has been shown to be required for its translation at maturation (2).…”

“…In Drosophila oogenesis, poly(A) tail lengthening occurs both at oocyte maturation and activation (3). Lengthening of the poly(A) tail of cyclin B mRNA at each of these meiotic transitions is correlated with renewed appearance of the protein (3,4), and polyadenylation of the mRNA for the meiosis-specific APC/C activator Cortex by the GLD2 cytoplasmic poly(A) polymerase has been shown to be required for its translation at maturation (2). Given these precedents, we tested whether polyadenylation could be an additional mechanism besides loss of Bruno contributing to the increase of Cyclin A protein at maturation.…”