Involved in the initiation of assembly of the COPII coat required for the formation of transport vesicles from the endoplasmic reticulum (ER) and the selection of cargo molecules. Also involved in autophagy. Interacts with SEC23, SEC31 and SED4. On the endoplasmic reticulum and on vesicles which bud from it. Present with 358 molecules/cell in log phase SD medium. Belongs to the SEC16 family.