Adds a GMP to the 5'-end of tRNA(His) after transcription and RNase P cleavage. This step is essential for proper recognition of the tRNA and for the fidelity of protein synthesis (Probable). Also functions as a guanyl-nucleotide exchange factor/GEF for the MFN1 and MFN2 mitofusins thereby regulating mitochondrial fusion (PubMed:25008184, PubMed:27307223). By regulating both mitochondrial dynamics and bioenergetic function, it contributes to cell survival following oxidative stress (PubMed:25008184, PubMed:27307223). a 5'-end ribonucleotide-tRNA(His) + ATP + GTP + H2O = a 5'-end phospho-guanosine-ribonucleotide-tRNA(His) + AMP + 2 diphosphate + H(+) Binds 2 magnesium ions per subunit. Homotetramer (PubMed:21059936). Interacts with MFN1 and MFN2; functions as a guanyl-nucleotide exchange factor/GEF for MFN2 and also probably MFN1 (PubMed:25008184). Expressed in many tissues. The disease is caused by variants affecting the gene represented in this entry. Belongs to the tRNA(His) guanylyltransferase family. Truncated N-terminus. Truncated N-terminus.