Mitofusin genes and encoded polypeptides are provided, including the Drosophila Fzo protein and its homologs from insects, other invertebrates, yeast, and vertebrates including mouse and humans. Mitofusins are large predicted GTPases with a predicted trans-membrane domain, coiled-coil regions, and a C-terminal region showing a high pI. The mitofusins are the first known protein mediator of mitochondrial fusion, and mediate developmentally regulated post-meiotic fusion of mitochondria. Missense mutations that alter conserved residues required for GTP binding in other GTPases inhibit the in vivo fusogenic activity of Fzo but do not affect its localization. Fusion proteins having amino acid sequences from mitofusin transmembrane regions localize to mitochondria. Mitofusins may be used in methods of identifying anti-insect and antifungal agents. Functional derivatives of mitofusins useful for such methods are described.