Fine-Tuning of mTOR mRNA and Nucleolin Complexes by SMN.

Francesca Gabanella,Carla Petrella,Claudio Passananti,Antonio Greco,Christian Barbato,Maria Grazia Di Certo,Marco De Vincentiis,Nicoletta Corbi,Antonio Minni,Marco Fiore

doi:10.3390/cells10113015

Francesca Gabanella, Carla Petrella + Show 8 more

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https://doi.org/10.3390/cells10113015

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Abstract

Increasing evidence points to the Survival Motor Neuron (SMN) protein as a key determinant of translation pathway. Besides its role in RNA processing and sorting, several works support a critical implication of SMN in ribosome biogenesis. We previously showed that SMN binds ribosomal proteins (RPs) as well as their encoding transcripts, ensuring an appropriate level of locally synthesized RPs. SMN impacts the translation machinery in both neural and non-neural cells, in agreement with the concept that SMN is an essential protein in all cell types. Here, we further assessed the relationship between SMN and translation-related factors in immortalized human fibroblasts. We focused on SMN-nucleolin interaction, keeping in mind that nucleolin is an RNA-binding protein, highly abundant within the nucleolus, that exhibits a central role in ribosomes production. Nucleolin may also affects translation network by binding the mammalian target of rapamycin (mTOR) mRNA and promoting its local synthesis. In this regard, for the first time we provided evidence that SMN protein itself associates with mTOR transcript. Collectively, we found that: (1) SMN coexists with nucleolin–mTOR mRNA complexes at subcellular level; (2) SMN deficiency impairs nucleolar compartmentalization of nucleolin, and (3) this event correlates with the nuclear retention of mTOR mRNA. These findings suggest that SMN may regulate not only structural components of translation machinery, but also their upstream regulating factors.

Highlights

Introduction published maps and institutional affilThe Survival Motor Neuron (SMN) protein was initially characterized once mutations in its coding gene, SMN1, were linked to motor neurons degeneration in spinal muscular atrophy (SMA) [1,2]
We found that SMN deficiency impairs nucleolar compartmentalization of nucleolin and this correlates with a nuclear retention of mammalian target of rapamycin (mTOR) mRNA
By a co-immunoprecipitation assay we showed that a pool of endogenous SMN protein co-precipitates with nucleolin in cellular extracts from immortalized human fibroblasts (Figure 1A)

Summary

Introduction

Introduction published maps and institutional affilThe Survival Motor Neuron (SMN) protein was initially characterized once mutations in its coding gene, SMN1, were linked to motor neurons degeneration in spinal muscular atrophy (SMA) [1,2]. SMN function in key processes such as resolution of R-loops/transcription termination, pre-mRNA splicing, and biogenesis of distinct ribonucleoprotein (RNP) complexes [7,8,9,10]. SMN appears to be involved in multiple facets of the translation pathway. It promotes intracellular trafficking and local translation of several mRNAs [2,11,12]. The SMN protein may target the mRNA by direct interaction, via its nucleic acid-binding domain, as well as by the association with distinct RNA-binding proteins [2,13,14]. SMN physically contacts translation machinery components and has been proposed as regulator of protein synthesis [15,16,17].

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