Coat protein complex II (COPII)-coated vesicles transport cargo proteins from the ER to the Golgi. ER-membrane-bound Sar1-GTP controls coat assembly on COPII-coated vesicles, whereas Sec23, the inner COPII coat protein, stimulates GTP hydrolysis by Sar1, dissociating Sar1 from the membrane. Understanding the precise spatiotemporal regulation of the Sar1-GTP to Sar1-GDP cycle and the membrane localisation of Sar1 is therefore of great importance. Now (p. 3231), Kazuo Kurokawa, Yasuyuki Suda and Akihiko Nakano use super-resolution confocal live microscopy to examine Sar1 localisation on COPII-coated membranes in living Saccharomyces cerevisiae cells. The authors show that Sar1 accumulates modestly around ER exit sites (ERES), where COPII-coated vesicles form, in a Sec16-dependent manner. Further, Sar1 accumulates at the rims of COPII-coated membranes but is excluded from the other areas of these membranes. The authors also show that GTP hydrolysis by Sar1 is required for restriction of Sar1 localisation to the rims of COPII-coated membranes. These results demonstrate that it is the reversible membrane association of Sar1 that restricts its localisation to the rim of COPII-coated membranes.
- © 2016. Published by The Company of Biologists Ltd