MIA2 (also known as CTAGE5), a transmembrane ER protein with roles in collagen secretion, recruits the SAR1 GEF PREB (also known as SEC12) to ER exit sites (Saito et al, 2014).
Compartment:
endoplasmic reticulum membrane,
integral component of lumenal side of endoplasmic reticulum membrane,
cytosol,
endoplasmic reticulum lumen
The cytoplasmic tails of MHC class I molecules do not possess any of the known ER export sequences. BAP31 acts as a cargo receptor by exporting peptide-loaded class I MHC molecules to the ER exit sites or ER/Golgi intermediate compartment. Sec24 of the Sec23-Sec24/Sar1-GTP complex catches cargo by direct contact, forming the prebudding complex. Sar1/Sec23-Sec24 can bring about curvature of the membrane in the formation of vesicle. The cargo (antigen-bound MHC) accumulates within the COPII vesicle by binding to the Sec24 polypeptide of the COPII coat.
SARS-CoV-2 ORF8 (8) induces BECN1-dependent autophagy to degrade class I MHC leading to impaired antigen presentation (Zhang Y et al. 2021).
Sec13-Sec31 binds the prebudding complex and this Sec13-Sec31 heterotetramer forms the outer structural scaffold of the coat. Sec13-Sec31 is likely to crosslink the prebudding complexes generating COPII-coated vesicles.
SAR1-GDP is recruited to the ER membrane by the transmembrane GEF (Guanine nucleotide exchange factor) PREB (also known as SEC12), where it is converted to SAR1-GTP. PREB itself is recruited to ER exit sites through interaction with MIA2 (also known as CTAGE5).