EMBO Fellows Meeting 2012

Robin Klemm
EMBO Fellows Meeting 2012
The molecular mechanism of homotypic ER fusion by the Atlastin and Sey1p
GTPases
Abstract
The endoplasmic reticulum (ER) is an essential organelle in all eukaryotic cells. It is comprised of the nuclear
envelope, peripheral ER sheets and a network of highly curved membrane tubules. The generation of the
network requires homotypic ER-ER fusion. In metazoans this is mediated by a class of dynamin-like GTPases
called atlastins (ATLs). Based on two crystal structures of the cytosolic segment of human ATL, biochemical
experiments and the in vitro reconstitution of the fusion reaction with the full-length protein, we propose a
molecular mechanism of homotypic ER fusion.
Plants and fungi do not have atlastin gene orthologs but they do contain potential functional orthologs called
RHD3 in A.thaliana and Sey1p in S.cerevisiae. Here, we show that the dynamin-like membrane bound GTPase
Sey1p mediates homotypic ER fusion in S.cerevisiae. The absence of Sey1p results in delayed ER fusion in vivo,
and proteoliposomes containing purified Sey1p fuse in a GTP dependent manner. Interestingly, human ATL can
replace Sey1p function in vivo. Like ATL, Sey1p undergoes GTP dependent dimerization and fusion is perturbed
by a mutation that in a plant homolog causes ER morphology defects. In addition, we find evidence for an
alternative ER-ER fusion pathway in S.cerevisiae which is dependent on the ER SNARE Ufe1p.
Taken together, our data show that Sey1p and its homologs function analogous to ATL and use a similar
molecular mechanism to mediate homotypic ER fusion.
Robin W. Klemm, Kamran Anwar, Xin Bian, Tina Y. Liu, Amanda Condon, Miao Zhang, Rodolfo Ghirlando, Xinqi Liu, Junjie
Hu, William A. Prinz, Tom A. Rapoport
14-17 June 2012, Heidelberg, Germany