This is the three-helix bundle (3HB) domain found at the C-terminal of Sey1 from yeast and RHD3 from Arabidopsis, GTP-binding proteins involved in homotypic membrane fusion of the endoplasmic reticulum [1,2,3]. This is a stalk-like domain composed of four 3HBs that interacts with its own GTPase domain and that of a neighbouring molecule, to undergo major conformational changes during the GTP cycle to drive membrane fusion [3].

1: The ROOT HAIR DEFECTIVE3 gene encodes an evolutionarily conserved protein with GTP-binding motifs and is required for regulated cell enlargement in Arabidopsis. Wang H, Lockwood SK, Hoeltzel MF, Schiefelbein JW; Genes Dev 1997;11:799-811. PMID:9087433

2: Function of a plant stress-induced gene, HVA22. Synthetic enhancement screen with its yeast homolog reveals its role in vesicular traffic. Brands A, Ho TH; Plant Physiol 2002;130:1121-1131. PMID:12427979

3: Structures of the yeast dynamin-like GTPase Sey1p provide insight into homotypic ER fusion. Yan L, Sun S, Wang W, Shi J, Hu X, Wang S, Su D, Rao Z, Hu J, Lou Z; J Cell Biol. 2015;210:961-972. PMID:26370501