GTPase tethering membranes through formation of trans-homooligomers and mediating homotypic fusion of endoplasmic reticulum membranes (PubMed:19665976). Functions in endoplasmic reticulum tubular network biogenesis (PubMed:19665976, PubMed:27619977). Localizes at endoplasmic reticulum (ER) three-way tubular junctions (PubMed:27619977). Belongs to the TRAFAC class dynamin-like GTPase superfamily. GB1/RHD3 GTPase family. GB1 subfamily.