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 (By similarity). The COPII coat is composed of at least 5 proteins: the SEC23/24 complex, the SEC13/31 complex, and the protein 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 (By similarity). Belongs to the WD repeat SEC31 family.