RNA-binding Motif Protein 4 Translocates to Cytoplasmic Granules and Suppresses Translation via Argonaute2 during Muscle Cell Differentiation

Jung-Chun Lin,Woan-Yuh Tarn

doi:10.1074/jbc.m109.032946

Abstract

The RNA-binding motif protein 4 (RBM4) plays multiple roles in mRNA metabolism, including translation control. RBM4 suppresses cap-dependent translation but activates internal ribosome entry site-mediated translation. Because of its high expression level in muscle and heart, we investigated the function of RBM4 in myoblast cells. Here, we demonstrate that RBM4 is phosphorylated and translocates to the cytoplasm in mouse C2C12 cells upon cell differentiation. Notably, RBM4 is transiently deposited into cytoplasmic granules containing microtubule assembly factors as well as poly(A)(+) RNAs. Moreover, RBM4 colocalizes with the components of micro-ribonucleoproteins, including the Argonaute2 (Ago2) protein, during muscle cell differentiation. RBM4 interacts directly with Ago2 and may recruit Ago2 to suppress translation of target mRNAs. Furthermore, RBM4 selectively associates with muscle cell-specific microRNAs and potentiates their translation repression activity by promoting micro-ribonucleoprotein association with target mRNAs. Altogether, our results suggest that RBM4 translocates to the cytoplasm and participates in translation suppression during muscle cell differentiation.

Highlights

In this study, we explore the function of RNA-binding motif protein 4 (RBM4) in muscle cells
We observed that RBM4 is colocalized with several centrosomal proteins and micro-RNP2 components in cytoplasmic granules of differentiated mouse C2C12 myoblasts
We found that RBM4 translocates to the cytoplasm most likely upon its phosphorylation, and it subsequently suppresses translation via binding to the CU-rich element(s) of target mRNAs and acts as a cofactor of miRNPs to potentiate their activity in translation suppression during muscle cell differentiation

Summary

Introduction

We explore the function of RBM4 in muscle cells. We observed that RBM4 is colocalized with several centrosomal proteins and micro-RNP (miRNP)2 components in cytoplasmic granules of differentiated mouse C2C12 myoblasts (see “Results”). We found that RBM4 translocates to the cytoplasm most likely upon its phosphorylation, and it subsequently suppresses translation via binding to the CU-rich element(s) of target mRNAs and acts as a cofactor of miRNPs to potentiate their activity in translation suppression during muscle cell differentiation. RBM4 Colocalizes with miRNP Components—Using in situ hybridization, we observed colocalization of poly(A)ϩ RNAs with RBM4 as well as pericentrin in differentiation-induced muscle cell granules (supplemental Fig. 2).

Results

Conclusion