Localization of Human RNase Z Isoforms: Dual Nuclear/Mitochondrial Targeting of the ELAC2 Gene Product by Alternative Translation Initiation

Abstract

RNase Z is an endonuclease responsible for the removal of 3′ extensions from tRNA precursors, an essential step in tRNA biogenesis. Human cells contain a long form (RNase ZL) encoded by ELAC2, and a short form (RNase ZS; ELAC1). We studied their subcellular localization by expression of proteins fused to green fluorescent protein. RNase ZS was found in the cytosol, whereas RNase ZL localized to the nucleus and mitochondria. We show that alternative translation initiation is responsible for the dual targeting of RNase ZL. Due to the unfavorable context of the first AUG of ELAC2, translation apparently also starts from the second AUG, whereby the mitochondrial targeting sequence is lost and the protein is instead routed to the nucleus. Our data suggest that RNase ZL is the enzyme involved in both, nuclear and mitochondrial tRNA 3′ end maturation.

RNase ZS1 processes UbL40 mRNAs and controls thermosensitive genic male sterility in rice

“…ELAC2 can process the lncRNA MALAT1 and plays a role in the generation of a mammalian herpesvirus microRNA 34,35 . ELAC1 (RNase Z S ) occurs in the cytoplasm but has unknown functions 36,37 . The enzymatic activity of ELAC2 is over 1,600-fold higher than ELAC1, suggesting that ELAC1 might have new functions other than tRNA processing in the cytoplasm 38 .…”

Partitioning of the Nuclear and Mitochondrial tRNA 3′-End Processing Activities between Two different Proteins in Schizosaccharomyces pombe

“…Metazoans appear to use alternative translation initiation sites. For example, in humans, the mitochondrial and nuclear forms of tRNase Z L can be generated from a single gene by the alternative use of two translation start codons (31,35). Because the first start codon of the human tRNase Z L gene is in a suboptimal context, some ribosomes would bypass it to initiate translation at a downstream AUG codon, thus generating essentially two identical tRNase Z L proteins, one mitochondrial and one nuclear.…”

“…Sequence analysis suggests that tRNase Z L , which is about twice the size of the short form, arises from gene duplication of tRNase Z S (26,27). tRNase Z L is present only in eukaryotes, whereas tRNase Z S exists primarily in prokaryotes, plants, and vertebrates (27)(28)(29) (30,31,33). In plants, the situation is more complex.…”

NSUN6 is a human RNA methyltransferase that catalyzes formation of m5C72 in specific tRNAs

“…ELAC2 was suggested to be involved in nuclear and mitochondrial tRNA 3 ′ end processing in vivo, whereas the cellular role of ELAC1 remains unclear (Rossmanith 2011). A subset of pre-tRNAs contains introns that are removed during nuclear tRNA splicing reactions in mammals (Paushkin et al 2004).…”