Mitochondria Export Sulfur Species Required for Cytosolic tRNA Thiolation

Abstract

In eukaryotes, mitochondria have been hypothesized to generate sulfur species required for tRNA thiolation in the cytosol, although no direct evidence thus far exists. Here we have detected these sulfur species, making use of our observation that isolated yeast cytosol alone is unable to thiolate tRNAs but can do so upon addition of mitochondria. Mitochondria were found to utilize the cysteine desulfurase Nfs1 to produce sulfur-containing species with masses ranging from 700 to 1,100 Da. Mitochondria exported these species via the Atm1 transporter in the inner membrane. Once exported to the cytosol, these sulfur species promoted cytosolic tRNA thiolation with no further requirement of mitochondria. Furthermore, we found that the Isu1/2 scaffolds but not the Ssq1 chaperone of the mitochondrial iron-sulfur cluster machinery were required for cytosolic tRNA thiolation, and thus the sulfur utilization pathway bifurcates at the Isu1/2 site for intra-organellar use in mitochondria or export to the cytosol.

Mitochondria export iron–sulfur and sulfur intermediates to the cytoplasm for iron–sulfur cluster assembly and tRNA thiolation in yeast

“…Here we show that mitochondria export an essential intermediate, called (Fe-S) int , to the cytoplasm that is then used for cytoplasmic iron-sulfur cluster assembly. However, the situation is even more complex, because another intermediate, a sulfur-only intermediate called S int , is made and exported by mitochondria and is required for cytoplasmic tRNA thiolation (39,40). The wobble uridines (U 34 ) of the cytoplasmic tRNAs specific for lysine, glutamate, and glutamine contain a thiomodification (41).…”

“…Note that the pathways for biosynthesis of cytoplasmic iron-sulfur clusters and cytoplasmic tRNA thiolation possess many intersecting and overlapping features. The sulfur-only S int , like the (Fe-S) int , is also made in mitochondria in a Nfs1-dependent manner and exported to the cytoplasm via Atm1 (39,40). However, these intermediates are distinct; they are not biochemically equivalent, and they are not interchangeable.…”

Naturally occurring modified ribonucleosides

“…Liu et al, 2014). While it is not known precisely which type of sulfur modification is directly replaced on uridine residues in eukaryotic tRNAs, mitochondrial NFS1, Isu1/2, and Atm1 in S. cerevisiae and Tum1, a persulfide mediator and activator of NFS1, have roles in forming and transporting s 2 U marks (Nakai et al, 2004;Noma, Sakaguchi, & Suzuki, 2009;Pandey et al, 2018). Moreover, the MTU1 enzyme is a mitochondrial-specific 2-thiouridylase that is a homolog of the MnmA enzyme (Black & Dos Santos, 2015;Noma et al, 2009).…”

A comprehensive mechanistic model of iron metabolism inSaccharomyces cerevisiae

“…27 Pain and coworkers discovered a 500-1000 Da sulfur-containing specie that is exported from intact mitochondria in an Atm1-dependent fashion. 29 They subsequently reported that mitochondria also export an Fe-S intermediate through Atm1 and presented evidence that the iron in the intermediate originates from mitochondria; 30 it's exciting to consider that this might be Cowan's [Fe 2 S 2 ] cluster.…”