Norvell A, Kelley R L, Wehr K, Schüpbach T
Howard Hughes Medical Insitute, Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544 USA.
Genes Dev. 1999 Apr 1;13(7):864-76. doi: 10.1101/gad.13.7.864.
Heterogeneous nuclear RNA-binding proteins, hnRNPs, have been implicated in nuclear export of mRNAs in organisms from yeast to humans. A germ-line mutation in a Drosophila hnRNP, Squid (Sqd)/hrp40, causes female sterility as a result of mislocalization of gurken (grk) mRNA during oogenesis. Alternative splicing produces three isoforms, SqdA, SqdB, and SqdS. Here we show that these isoforms are not equivalent; SqdA and SqdS perform overlapping but nonidentical functions in grk mRNA localization and protein accumulation, whereas SqdB cannot perform these functions. Furthermore, although all three Sqd isoforms are expressed in the germline cells of the ovary, they display distinct intracellular distributions. Both SqdB and SqdS are detected in germ-line nuclei, whereas SqdA is predominantly cytoplasmic. We show that this differential nuclear accumulation is correlated with a differential association with the nuclear import protein Transportin. Finally, we provide evidence that grk mRNA localization and translation are coupled by an interaction between Sqd and the translational repressor protein Bruno. These results demonstrate the isoform-specific contributions of individual hnRNP proteins in the regulation of a specific mRNA. Moreover, these data suggest a novel role for hnRNPs in localization and translational regulation of mRNAs.
异质性核RNA结合蛋白（hnRNPs）在从酵母到人类的生物体中参与mRNA的核输出过程。果蝇的一种hnRNP——鱿鱼（Sqd）/hrp40发生种系突变，导致卵子发生过程中gurken（grk）mRNA定位错误，从而造成雌性不育。可变剪接产生三种亚型，即SqdA、SqdB和SqdS。我们在此表明，这些亚型并不等同；SqdA和SqdS在grk mRNA定位和蛋白质积累过程中发挥重叠但不完全相同的功能，而SqdB无法执行这些功能。此外，尽管所有三种Sqd亚型都在卵巢的生殖细胞中表达，但它们呈现出不同的细胞内分布。SqdB和SqdS都在生殖细胞核中被检测到，而SqdA主要位于细胞质中。我们发现这种不同的核积累与和核输入蛋白转运蛋白的不同结合有关。最后，我们提供证据表明，grk mRNA的定位和翻译通过Sqd与翻译阻遏蛋白布鲁诺之间的相互作用而偶联。这些结果证明了单个hnRNP蛋白在特定mRNA调控中的亚型特异性作用。此外，这些数据表明hnRNPs在mRNA的定位和翻译调控中具有新的作用。
