Small G Proteins: Arf Family GTPases in Vesicular Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 195

Fig. 9.2 Individual steps in the live cycle of COPII vesicles. Sar1 is recruited and anchored to the
ER membrane by activation via the GEF Sec12. Membrane-bound Sar1 recruits the inner COPII
coat subcomplex Sec23/24 by binding to the Sec23 subunit. The cargo binding subunit Sec24
binds ER export motif-containing transmembrane cargo proteins and trafficking proteins
(e.g. SNAREs), as well as cargo adaptors (e.g. ERGIC53 and p24 proteins). The outer COPII
subcomplex Sec13/31 binds to the pre-budding complexes and drives coat polymerisation. Vesicle
scission requires the membrane curvature potentiating activity of the small GTPase Sar1. Sec23-
stimulated GTP hydrolysis by Sar1 leads to release of the small GTPase and concomitant release of
the coat
In addition, membrane cargo seems also to contribute to COPII coat stability by
increasing the membrane association time of the COPII coat during multiple cycles
of GTP hydrolysis (Forster et al. 2006 ; Sato and Nakano 2005 ). This may represent