BTV NS4 shows strong relatedness to fzo-mitofusin protein, a putative transmembrane GTPase. The fzo protein has a coiled-coil structure and mediates mitochondrial fusion [36] . Another protein family with a coiled-coil structure, which also shows a strong match with BTV NS4, is EMP24_GP25L. Members of this family have been implicated in transporting 'cargo' from the endoplasmic reticulum (ER) and are related to the previously described GOLD domain [37] , which is always found combined with lipid-or membrane-association domains.