<p>The GTP-binding domain of all TrmE/ThdF orthologues is found in the C-terminal portion of the molecule. The N-terminal half can be removed without affecting the GTP-binding/hydrolysis function of the GTP-binding domain. The last four amino acids of all orthologues of ThdF/TrmE are highly conserved, being either CIGK or CLGK. This matches the Caax (where 'a' represents an aliphatic amino acid, and 'x' represents any amino acid) motif for isoprenylation that anchors small GTP-binding proteins to cell membranes in eukaryotic cells. However, protein isoprenylation has never been shown to occur in bacteria. Interestingly, biochemical experiments have shown that the <taxon tax_id="562">Escherichia coli</taxon> TrmE protein peripherally associates with the membrane fraction [<cite idref="PUB00007679"/>].</p> <p> Although the biochemical properties of TrmE have been investigated for the E. coli and <taxon tax_id="2336">Thermotoga maritima</taxon> proteins, nothing is known about the relationship of this protein to tRNA modification. Orthologues of TrmE are present in eukaryotes and bacteria, but are not present in archaea. In <taxon tax_id="4932">Saccharomyces cerevisiae</taxon>, Mss1p is a nuclear-encoded mitochondrial protein that is the yeast orthologue of TrmE. Mss1p interacts with the 15S rRNA of the yeast mitochondria, which is equivalent to the 16S rRNA of bacteria. Subsequent analysis of the S. cerevisiae MTO1 gene suggests that MSS1 and MTO1 act together in a pathway involved in optimizing mitochondrial protein synthesis. </p> <p>TrmE may play a role in tRNA processing and may be directly or indirectly involved in regulating ribosome function.</p>