Talk Abstracts

Talk on Friday 02:15-02:30pm submitted by Kunal Chatterjee

Identification of novel proteins involved in nuclear export of pre-tRNAs in Saccharomyces cerevisiae

Kunal Chatterjee (Department of Molecular Genetics, The Ohio State University), Jingyan Wu (Department of Genetics, Stanford University), Yao Wan (Department of Molecular Genetics, The Ohio State University), Anita Hopper (Department of Molecular Genetics, The Ohio State University)

Abstract:
tRNAs perform essential role of delivering amino acids to the cytoplasmic protein synthesis machinery. To execute this role, eukaryotic tRNAs must be escorted out of the nucleus, their site of synthesis, to the cytoplasm, their site of function. Via primary nuclear export, newly transcribed, end-matured, partially modified tRNAs are shuttled to the cytoplasm. Genetic and biochemical studies have identified a set of RanGTPase binding proteins called β-importins implicated in this tRNA movement. Recent in vivo biochemical data in yeast have shown two such members of β-importin family, Los1 (Exportin-t in vertebrates) and Msn5 (Exportin 5), serve overlapping but distinct functions in tRNA export¹. Although Los1 and Msn5 both participate in tRNA nuclear export, they cannot be the only nuclear exporters for tRNAs as los1Δ msn5Δ double mutant cells are viable². An unbiased screening representing ~90% of the total yeast proteome conducted recently in our laboratory uncovered novel genes involved in tRNA subcellular localization³. Two genes MEX67 and MTR2 when inactivated, cause accumulation of end-matured unspliced tRNAs in the nucleus as determined by FISH and Northern hybridization assays. Moreover, concurrent overexpression of Mex67 and Mtr2 suppresses tRNA export defects associated with los1Δ. Interestingly, the Mex67-Mtr2 complex is the principal yeast nuclear export factor for bulk mRNA⁴ and also contributes to ribosomal subunit nuclear export⁵. Hence, it is possible that this complex binds tRNAs as well and may constitute a missing alternate pathway for nuclear export. The total proteome screening also uncovered Crm1 to be involved in tRNA nuclear export. Crm1, which functions in nuclear export of proteins with Leucine rich motifs and pre-ribosomal particles⁶, may provide yet another pathway for tRNAs to be transported from the nucleus to the cytoplasm. Our studies lead to the hypothesis that tRNAs may highjack both the mRNA and protein nuclear export machineries as alternative pathways for tRNA nuclear export.

References:
1. Huang H.-Y.,Hopper A.K . In vivo biochemical analyses reveal distinct roles of β-importins and eEF1A in tRNA subcellular traffic. Genes Dev. 29:772-783 (2015).
2. Hopper A.K. tRNA post-transcriptional processing, turnover, and subcellular dynamics in the yeast Saccharomyces cerevisiae. Genetics 194:43-67 (2013).
3. Wu J, Bao A, Chatterjee K, Wan Y, Hopper A.K. Genome-wide screen uncovers novel pathways for tRNA processing and nuclear-cytoplasmic dynamics (in revision).
4. Okamura M, Inose H, Masuda S. RNA Export through the NPC in Eukaryotes. Genes (Basel). 6:124-49 (2015).
5. Faza MB, Chang Y, Occhipinti L, Kemmler S, Panse VG.Role of Mex67-Mtr2 in the Nuclear Export of 40S Pre-Ribosomes. PLoS Genet 8(8) (2012).
6. Tschochner H, Hurt E. Pre-ribosomes on the road from the nucleolus to the cytoplasm. TRENDS in Cell Biology.13: 255–263 (2003).

Keywords: tRNA export, Mex67-Mtr2, Crm1