Mitochondrial outer membrane GTPase that mediates mitochondrial clustering and fusion (PubMed:12475957, PubMed:12759376, PubMed:27920125, PubMed:28114303). Membrane clustering requires GTPase activity (PubMed:27920125). It may involve a major rearrangement of the coiled coil domains (PubMed:27920125, PubMed:28114303). Mitochondria are highly dynamic organelles, and their morphology is determined by the equilibrium between mitochondrial fusion and fission events (PubMed:12475957, PubMed:12759376). Overexpression induces the formation of mitochondrial networks (in vitro) (PubMed:12759376). Has low GTPase activity (PubMed:27920125, PubMed:28114303). GTP + H2O = GDP + H(+) + phosphate Homodimer, also in the absence of bound GTP (PubMed:27920125, PubMed:28114303). Forms higher oligomers in the presence of a transition state GTP analog (PubMed:28114303). Forms homomultimers and heteromultimers with MFN2 (By similarity). Oligomerization is essential for mitochondrion fusion (PubMed:27920125, PubMed:28114303). Component of a high molecular weight multiprotein complex (PubMed:12759376). Interacts with VAT1 (By similarity). Interacts with THG1L; THG1L probably functions as a guanyl-nucleotide exchange factor/GEF, activating MFN1. Detected in kidney and heart (at protein level) (PubMed:12759376). Ubiquitous (PubMed:11950885, PubMed:12759376). Expressed at slightly higher level in kidney and heart (PubMed:12759376). Isoform 2 may be overexpressed in some tumors, such as lung cancers (PubMed:11751411). 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 (PubMed:23933751). Deubiquitination by USP30 inhibits mitochondrial fusion (By similarity). Ubiquitinated by MARCHF5 (PubMed:20103533). 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 (PubMed:23933751). A truncated MFN1 construct containing the GTPase domain and the associated helix bundle is a monomer in the absence of bound GTP and a homodimer in the GTP-bound form; GDP cannot replace GTP and induce dimerization. Belongs to the TRAFAC class dynamin-like GTPase superfamily. Dynamin/Fzo/YdjA family. Mitofusin subfamily.