As a component of the coat protein complex II (COPII), may function in vesicle budding and cargo export from the endoplasmic reticulum. 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 (By similarity). Interacts with SEC31A. Ubiquitously expressed at low levels with specific expression in thymus and testis. Expressed in testis by Sertoli cells, Leydig cells and spermatogonia and in cerebellum more prominently by Purkinje and granular cells (at protein level). Monoubiquitinated by the BCR(KLHL12) E3 ubiquitin ligase complex, leading to regulate the size of COPII coats. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the WD repeat SEC31 family.