Component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER). The coat has two main functions, the physical deformation of the endoplasmic reticulum membrane into vesicles and the selection of cargo molecules. COPII is composed of at least 5 proteins: the SEC23/24 complex, the SEC13/31 complex and SAR1. SEC13 and SEC31 make a 2:2 tetramer that forms the edge element of the COPII outer coat. The tetramer self-assembles in multiple copies to form the complete polyhedral cage. Interacts (via WD 8) with SEC13 (PubMed:10574704, PubMed:10788476). Interacts with PDCD6; interaction takes place in response to cytosolic calcium increase and leads to bridge together the BCR(KLHL12) complex and SEC31A, leading to monoubiquitination. Interacts with KLHL12 (By similarity). Associates with membranes in a GTP-dependent manner (PubMed:10574704). Localizes to endoplasmic reticulum exit sites (ERES), also known as transitional endoplasmic reticulum (tER) (By similarity). Ubiquitously expressed. The ALG-2-binding site motif-2 (ABS-2) contains a PXPGF sequence that binds hydrophobic pocket 3 of PDCD6. Monoubiquitinated by the BCR(KLHL12) E3 ubiquitin ligase complex, leading to regulate the size of COPII coats. Belongs to the WD repeat SEC31 family.