A proline-rich protein binds to the localization element of Xenopus Vg1 mRNA and to ligands involved in actin polymerization

Abstract

A 340 nucleotide element within the 3¢ untranslated region of Vg1 mRNA determines its localization to the vegetal cortex of Xenopus oocytes. To identify protein factors that bind to this region, we screened a cDNA expression library with an RNA probe containing this sequence. Five independent isolates encoded a protein (designated Prrp for proline-rich RNA binding protein) having two RNP domains followed by multiple polyproline segments. Prrp and Vg1 mRNAs are co-localized to the vegetal cortex of stage IV oocytes, substantiating an interaction between the two in vivo. Prrp also associates with VegT mRNA, which like Vg1 mRNA uses the late localization pathway, but not with Xcat-2 or Xwnt-11 mRNAs, which use the early pathway. The proline-rich domain of Prrp interacts with pro®lin, a protein that promotes actin polymerization. Prrp can also associate with the EVH1 domain of Mena, another micro®lament-associated protein.Since the anchoring of Vg1 mRNA to the vegetal cortex is actin dependent, one function of Prrp may be to facilitate local actin polymerization, representing a novel function for an RNA binding protein.

The DAZL and PABP families: RNA-binding proteins with interrelated roles in translational control in oocytes

“…Human DAZAP-1 is a nucleocytoplasmic shuttling protein (Lin & Yen 2006) and its X. laevis ortholog, PRRP, associates with mRNA localization elements in Vg1 and VegT mRNAs (Zhao et al 2001). However, it is unclear whether DAZAP-1 plays an active role in mRNA localization and its role on these mRNAs appears to be independent of xDAZL.…”

The KH domains of Xenopus Vg1RBP mediate RNA binding and self-association

“…Vg1RBP (also known as Vera) is a protein implicated in localizing Vg1 mRNA to the vegetal cortex during Xenopus oogenesis (Deshler et al+, 1998;Havin et al+, 1998;Zhang et al+, 1999;Kwon et al+, 2002)+ One of the bestcharacterized localized maternal RNAs in Xenopus, Vg1 mRNA encodes a TGF-b-like factor involved in mesoderm induction and left-right axis formation+ Vg1 mRNA translocation to and anchoring at the vegetal cortex involves microtubules and microfilaments, respectively, and is mediated by the 340-nt 39 UTR vegetal localization element (VLE) defined in microinjection assays (Yisraeli et al+, 1990;Mowry & Melton, 1992; reviewed in King et al+, 1999;Palacios & St+ Johnston, 2001;Yaniv & Yisraeli, 2001;Kloc et al+, 2002)+ Vg1 mRNA is not translated until the mRNA is localized, reflecting the tight control of RNA translation and localization shown to be critical for pattern formation in Drosophila (Palacios & St+ Johnston, 2001;Kloc et al+, 2002)+ The 250-nt UA-rich element that represses Vg1 mRNA translation is also located in the 39 UTR, distinct from and downstream of the VLE (Wilhelm et al+, 2000;Otero et al+, 2001)+ Several proteins interact specifically with the VLE (Mowry, 1996), including Vg1RBP/Vera (Deshler et al+, 1998;Havin et al+, 1998;Zhang et al+, 1999), hnRNP I/PTB (Cote et al+, 1999), and PrrP (Zhao et al+, 2001), all of which colocalize with Vg1 mRNA and are thought to play a role in the localization process+ Vg1RBP/Vera and hnRNP I bind the pyrimidine-rich VM1 and E2 motifs which are repeated three to five times throughout the VLE; mutation or deletion of these elements abolishes protein-binding and RNA localization (Deshler et al+, 1998;Havin et al+, 1998;Cote et al+, 1999;Bubunenko et al+, 2002;Kwon et al+, 2002)+ Vg1RBP mediates the association of Vg1 RNA with microtubules in Xenopus oocytes (Elisha et al+, 1995), whereas PrrP has been reported to bind to ligands involved in actin polymerization (Zhao et al+, 2001)+ Although these data strongly implicate these proteins in mediating Vg1 mRNA localization, the actual binding sites of Vg1RBP/ hnRNP I/P...…”

Participation of Xenopus Elr-type Proteins in Vegetal mRNA Localization during Oogenesis

“…LEs recruit proteins to form a localization complex. Although proteins that exclusively interact with LEs from early localizing RNAs and that could mediate the entrapment in the MC have not been identified to date, a number of proteins that interact with the localization element of the late localizing Vg1 mRNA have been identified; they include Vg1RBP, hnRNP I, Prrp, VgRBP71/KSRP, XStaufen 1, and 40LoVe (15,(22)(23)(24)(25)(26)(27). Interestingly, mitochondrial cloud localization elements of all early pathway RNAs tested to date can enter the late localization pathway if injected into stage III/IV oocytes, suggesting that they are able to recruit late transport proteins (17, 28 -31).…”

“…VgRBP71/KSRP and 40LoVe can also be detected in the nucleus, but whether they are indeed part of a nuclear transport RNP remains to be determined (26,27). The reassembly step in the cytoplasm includes the recruitment of additional proteins; whereas hnRNP I, Vg1RBP, Prrp, XStaufen 1, and 40LoVe accompany the localizing RNA in the vegetal cytoplasm and get enriched at the cortex (15,25,27,32,34,35), VgRBP71/KSRP is found throughout the cytoplasm with a slight enrichment at the animal cortex (26). Rather than directly participating in the vegetal transport, VgRBP71/KSRP has been suggested to translationally activate cortical Vg1 mRNA by stimulating a nuclease that cleaves off the Vg1 translational control element (TCE) (36).…”

“…Rather than directly participating in the vegetal transport, VgRBP71/KSRP has been suggested to translationally activate cortical Vg1 mRNA by stimulating a nuclease that cleaves off the Vg1 translational control element (TCE) (36). Because of its interaction with profilin, a regulator of actin dynamics, Prrp has been proposed to function in the microfilament-dependent anchoring of localized RNA at the cortex (25). The recruitment of Staufen 1 into the transport particle might be mediated by hnRNP I, because dominant negative Staufen 1 mutants not only affect vegetal localization of injected RNAs but also lose interaction with hnRNP I (15).…”