COPII is a coat protein complexe. Cargo proteins exiting the endoplasmic reticulum en route to the Golgi are typically carried in 60-70 nm vesicles surrounded by the COPII protein coat.

Some secretory cargo assemblies in specialized mammalian cells are too large for transport within such carriers.

Recent studies on procollagen-I and chylomicron trafficking have reached conflicting conclusions regarding the role of COPII proteins in ER exit of these large biological assemblies.

COPII is no doubt essential for such transport in vivo, but it remains unclear whether COPII envelops the membrane surrounding large cargo or instead plays a more indirect role in transport carrier biogenesis.

See also:

 COPI
 SARA2 (germline mutations in Anderson disease and chylomicron retention disease) from SAR1 family of proteins

• SARAs family of proteins initiates the intracellular transport of proteins in COPII (coat protein)-coated vesicles.
• Chylomicrons, which vastly exceed the size of typical COPII vesicles, are selectively recruited by the COPII machinery for transport through the secretory pathways of the enterocytic cell.

References

 Fromme JC, Schekman R. COPII-coated vesicles: flexible enough for large cargo? Curr Opin Cell Biol. 2005 Aug;17(4):345-52. PMID: #15975775#

 Palmer KJ, Stephens DJ. Biogenesis of ER-to-Golgi transport carriers: complex roles of COPII in ER export. Trends Cell Biol. 2004 Feb;14(2):57-61. PMID: #15106609#

 Jones B, Jones EL, Bonney SA, Patel HN, Mensenkamp AR, Eichenbaum-Voline S, Rudling M, Myrdal U, Annesi G, Naik S, Meadows N, Quattrone A, Islam SA, Naoumova RP, Angelin B, Infante R, Levy E, Roy CC, Freemont PS, Scott J, Shoulders CC. Mutations in a Sar1 GTPase of COPII vesicles are associated with lipid absorption disorders. Nat Genet. 2003 May;34(1):29-31. PMID: #12692552#