Mitochondrial outer membrane GTPase that mediates mitochondrial clustering and fusion (PubMed:14561718, PubMed:12589796). Membrane clustering requires GTPase activity. It may involve a major rearrangement of the coiled coil domains (By similarity). Mitochondria are highly dynamic organelles, and their morphology is determined by the equilibrium between mitochondrial fusion and fission events (PubMed:14561718). Overexpression induces the formation of mitochondrial networks (in vitro) (PubMed:14561718). Has low GTPase activity (By similarity). GTP + H2O = GDP + H(+) + phosphate Homodimer, also in the absence of bound GTP. Forms higher oligomers in the presence of a transition state GTP analog (By similarity). Forms homomultimers and heteromultimers with MFN2 (PubMed:14561718). Oligomerization is essential for mitochondrion fusion. Component of a high molecular weight multiprotein complex (By similarity). Interacts with VAT1 (PubMed:17105775). Interacts with THG1L; THG1L probably functions as a guanyl-nucleotide exchange factor/GEF, activating MFN1. Ubiquitous. In brain, it is expressed at weaker level than MFN2, while it is expressed at a higher level than MFN2 in heart and testis. Expressed at high level in elongating spermatids of seminiferous tubules. A helix bundle is formed by helices from the N-terminal and the C-terminal part of the protein. The GTPase domain cannot be expressed by itself, without the helix bundle. Rearrangement of the helix bundle and/or of the coiled coil domains may bring membranes from adjacent mitochondria into close contact, and thereby play a role in mitochondrial fusion. Ubiquitinated by non-degradative ubiquitin by PRKN (By similarity). Deubiquitination by USP30 inhibits mitochondrial fusion (By similarity). Ubiquitinated by MARCHF5. When mitochondria are depolarized and dysfunctional, it is ubiquitinated by a SCF (SKP1-CUL1-F-box protein) E3 ubiquitin-protein ligase complex that contains FBXO7 and PRKN (By similarity). Belongs to the TRAFAC class dynamin-like GTPase superfamily. Dynamin/Fzo/YdjA family. Mitofusin subfamily.