Essential transmembrane GTPase, which mediates mitochondrial fusion during spermatogenesis. In early spermatocytes, fusion of mitochondria give rise to two organelles named Nebenkern and constitutes an important step in mitochondria morphology, which is balanced between fusion and fission. Essential for fertility (By similarity). GTP + H2O = GDP + H(+) + phosphate Belongs to the TRAFAC class dynamin-like GTPase superfamily. Dynamin/Fzo/YdjA family. Mitofusin subfamily.