Required for efficient protein export from the endoplasmic reticulum (ER) to the Golgi by regulating COPII coat dynamics at the ER. Functions as a scaffold and regulator of COPII coat assembly at ER exit sites. Interacts with SEC13A, SEC13B and SEC31A. Localizes at Golgi-associated cup-shaped endoplasmic reticulum exit sites. No visible phenotype under normal growth conditions, but dry seeds of mutant plants accumulate the precursors of the two major storage proteins albumin 2S and globulin 12S. Belongs to the SEC16 family.