Nucleoporin TPR promotes tRNA nuclear export and protein synthesis in lung cancer cells

Miao Chen, Qian Long, Melinda S. Borrie, Haohui Sun, Changlin Zhang, Han Yang, Dingbo Shi, Marc R. Gartenberg, Wuguo Deng

Abstract
The robust proliferation of cancer cells requires vastly elevated levels of protein synthesis, which relies on a steady supply of aminoacylated tRNAs. Delivery of tRNAs to the cytoplasm is a highly regulated process, but the machinery for tRNA nuclear export is not fully elucidated. In this study, using a live cell imaging strategy that visualizes nascent transcripts from a specific tRNA gene in yeast, we identified the nuclear basket proteins Mlp1 and Mlp2, two homologs of the human TPR protein, as regulators of tRNA export. TPR expression is significantly increased in lung cancer tissues and correlated with poor prognosis. Consistently, knockdown of TPR inhibits tRNA nuclear export, protein synthesis and cell growth in lung cancer cell lines. We further show that NXF1, a well-known mRNA nuclear export factor, associates with tRNAs and mediates their transport through nuclear pores. Collectively, our findings uncover a conserved mechanism that regulates nuclear export of tRNAs, which is a limiting step in protein synthesis in eukaryotes.

Introduction
Protein synthesis levels are tightly controlled to match the demands of cell growth. The demand for increased protein synthesis is greatest during the uncontrolled proliferation of cancer cells. Thus, understanding and harnessing the rate limiting steps in protein synthesis represents an important goal in cancer research. Studies have shown that the cellular tRNA pool plays an important role in the regulation of protein synthesis, and a role for tRNA abundance in cancer is emerging [1]. Not only is the size of the tRNA pool different in cancer cells but the content of specific tRNAs in that pool adjusts to selectively increase expression of genes required for cancer cell growth [2]. Differences in transcription of a subset of tRNA genes appear to drive the differences in tRNA content between cancerous and differentiated cells [3].

Results
Previously, we found that tRNA genes dock at NPCs when their expression peaks in M phase [26]. The behavior was reminiscent of several well-characterized RNA polymerase II-transcribed genes [27], and suggested that docking at NPCs coordinated tRNA production with nuclear export. Association of tRNA genes at NPCs required several factors, including nucleoporin Nup60 and nascent tRNA exportin Los1. Nup60 binds stably to the nuclear basket and anchors yeast myosin-like protein 1 (Mlp1) to the nucleoplasmic side of the NPC [28,29]. Mlp1 and its paralog Mlp2 are orthologs of the TPR protein in higher eukaryotes. The early finding that Mlp proteins associated with a subset of active RNA polymerase II-transcribed genes placed them at the nexus of transcription and NPC contact [30].

Discussion
The major findings of our current study are that (i) the yeast homologs of TPR, Mlp1 and Mlp2, are required for tRNA gene docking at NPCs and nuclear export of nascent tRNA transcripts; (ii) the tRNA export role for TPR is conserved in human lung cancer cells, and fulfilled through association with NXF1, an RNA binding factor that mediates export of both tRNA and mRNA through NPCs; (iii) TPR coordinates the nuclear export of various RNAs including mRNAs, tRNAs and rRNAs, and promotes protein synthesis in eukaryotes.

Citation: Chen M, Long Q, Borrie MS, Sun H, Zhang C, Yang H, et al. (2021) Nucleoporin TPR promotes tRNA nuclear export and protein synthesis in lung cancer cells. PLoS Genet 17(11): e1009899. https://doi.org/10.1371/journal.pgen.1009899

Editor: Ben Montpetit, University of California Davis, UNITED STATES

Received: April 7, 2021; Accepted: October 21, 2021; Published: November 18, 2021

Copyright: © 2021 Chen et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: This work was supported by National Natural Science Foundation of China (81702761 and 81972623 [MC], 81772925 and 81972569 [WD], http://www.nsfc.gov.cn/english/site_1/index.html), Guangdong Basic and Applied Basic Research Foundation (2020A1515010253 [MC], http://gdstc.gd.gov.cn/), United States Public Health Service Grants (NIGMS 51402 [MRG], https://www.nigms.nih.gov/) and New Jersey Department of Health (DFHS15PPC036 [MC], https://www.nj.gov/health/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Latest Issue
Get instant
access to our latest e-book
Sanofi UpperEdge - Healthcare ERP Transformation Medical Manufacturing Asia 2022 Medical Fair Asia 2022