Transport vesicle coat protein, Sec13/31 complex
Transport vesicles export newly synthesized proteins from endoplasmic reticulum (ER) to the other organelles. The coat protein complex II (COPII) coated vesicle is the one that shuttles from ER to Goldi. COPII consists of a small GTPase Sar1, Sec23 and Sec24 complex (Sec23/24), where Sec23 is a Sar1-specific GTPase-activating protein and Sec24 functions in cargo selection, and Sec13 and 31 complex (Sec13/31), which is the main component of the coat structural proteins. Sec13/31 can self-assemble to form the cage on the surface of transport vesicle. The analysis with cryo-electron microscopy (cryo-EM) revealed that Sec13/31 cage has the flexible cuboctahedron geometry.
fig1. Domain structure of Sec13/31 is mapped on the sequences.
Full-length Sec13 forms β propeller domain whereas in the Sec31, residues 1-365 forms β propeller domain, residues 370-762 and C-terminus form α solenoid domain, and residues 770-1110 forms flexible proline rich region.
fig2. Structure of Sec13/31-Sec31/13 heterotetramer
In fig2, Sec13/31-Sec31/13 heterotetramer which contains full-length β propeller of Sec13 and central α solenoid of Sec31 (residues 370-762) is shown. This is an edge of the cargo cuboctahedron. Two Sec13/31 asymmetrically assemble via central α solenoids with two β propellers at each end.
fig3. Cuboctahedron coat formed by Sec13/31-Sec31/13 heterotetramers
In fig3, the Sec13/31-Sec31/13 heterotetramers are fitted in the cuboctahedron (the missing domains in the fig2 are inserted here). This structure explains the lower integrity and the flexibility of COPII coat. Converged four β propellers of Sec31 on a vertex without assembling show the weak interaction between the edges. The central α solenoids assembling like a hinge makes the COPII cage flexible and may enable the cage to accommodate various-sized vesicles.
Protein Data Bank (PDB)
Fath, S. Mancias, J.D. Bi, X. Goldberg, J.; "Structure and Organization of Coat Proteins in the COPII Cage."; Cell(Cambridge,Mass.); (2007) 129:1325-1336 PubMed:17604721.
author: Daisuke Ino