RAP55, a Cytoplasmic mRNP Component, Represses Translation in Xenopus Oocytes

Kimio J. Tanaka,Ken Matsumoto,Naoko Imamoto,Masafumi Tsujimoto,Masatoshi Takagi,Kenji Ogawa

doi:10.1074/jbc.m609059200

Kimio J. Tanaka, Ken Matsumoto + Show 4 more

Open Access

https://doi.org/10.1074/jbc.m609059200

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Abstract

mRNAs in eukaryotic cells are presumed to always associate with a set of proteins to form mRNPs. In Xenopus oocytes, a large pool of maternal mRNAs is masked from the translational apparatus as storage mRNPs. Here we identified Xenopus RAP55 (xRAP55) as a component of RNPs that associate with FRGY2, the principal component of maternal mRNPs. RAP55 is a member of the Scd6 or Lsm14 family. RAP55 localized to cytoplasmic foci in Xenopus oocytes and the processing bodies (P-bodies) in cultured human cells: in the latter cells, RAP55 is an essential constituent of the P-bodies. We isolated xRAP55-containing complexes from Xenopus oocytes and identified xRAP55-associated proteins, including a DEAD-box protein, Xp54, and a protein arginine methyltransferase, PRMT1. Recombinant xRAP55 repressed translation, together with Xp54, in an in vitro translation system. In addition, xRAP55 repressed translation in oocytes when tethered with a reporter mRNA. Domain analyses revealed that the N-terminal region of RAP55, including the Lsm domain, is important for the localization to P-bodies and translational repression. Taken together, our results suggest that xRAP55 is involved in translational repression of mRNA as a component of storage mRNPs.

Highlights

Growing evidence indicates that mRNAs move between nontranslating and translating messenger ribonucleoproteins (mRNPs) states in the cytoplasm in germ cells and in cell types as diverse as neurons and yeast [2,3,4]
We show that xRAP55 and its human homologue localize to cytoplasmic foci in oocytes and cultured cells and that xRAP55 is a component of storage mRNPs and acts as a translational repressor
Identification of the Components of FRGY2-associated Complexes—Given that FRGY2 is one of the major components of Xenopus maternal mRNPs, we attempted to search for proteins that interact with FRGY2 in oocytes

Summary

Introduction

Growing evidence indicates that mRNAs move between nontranslating (stored) and translating mRNP states in the cytoplasm in germ cells and in cell types as diverse as neurons and yeast [2,3,4]. The amount and activity of the Y-box protein, a core component of cytoplasmic mRNPs, have been shown to significantly affect general translational activity in Xenopus oocytes and in somatic cells of different organisms [14, 15]. Y-box proteins FRGY2 and mRNP3 were identified as major RNA-binding components of storage mRNPs in Xenopus oocytes [16, 17]. Later studies identified other components of mRNPs, including a DEAD-box ATPase Xp54 and embryonic poly(A)binding proteins, ePAB and ePABP2 [23,24,25,26,27]. We show that xRAP55 and its human homologue localize to cytoplasmic foci in oocytes and cultured cells and that xRAP55 is a component of storage mRNPs and acts as a translational repressor

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