Human polypyrimidine tract-binding protein interacts with mitochondrial tRNAThr in the cytosol

摘要

Human polypyrimidine tract-binding protein PTB is a multifunctional RNA-binding protein with four RNA recognition motifs (RRM1 to RRM4). PTB is a nucleocytoplasmic shuttle protein that functions as a key regulator of alternative pre-mRNA splicing in the nucleoplasm and promotes internal ribosome entry site-mediated translation initiation of viral and cellular mRNAs in the cytoplasm. Here, we demonstrate that PTB and its paralogs, nPTB and ROD1, specifically interact with mitochondrial (mt) tRNA^Thr both in human and mouse cells. In vivo and in vitro RNA-binding experiments demonstrate that PTB forms a direct interaction with the T-loop and the D-stem-loop of mt tRNA^Thr using its N-terminal RRM1 and RRM2 motifs. RNA sequencing and cell fractionation experiments show that PTB associates with correctly processed and internally modified, mature mt tRNA^Thr in the cytoplasm outside of mitochondria. Consistent with this, PTB activity is not required for mt tRNA^Thr biogenesis or for correct mitochondrial protein synthesis. PTB association with mt tRNA^Thr is largely increased upon induction of apoptosis, arguing for a potential role of the mt tRNA^Thr/PTB complex in apoptosis. Our results lend strong support to the recently emerging conception that human mt tRNAs can participate in novel cytoplasmic processes independent from mitochondrial protein synthesis.