Regulator of endoplasmic reticulum secretion that acts as a key determinant of brain size. Required for secretion of extracellular matrix proteins. Required for correct brain development by depositing sufficient extracellular matrix proteins for tissue integrity and the proliferation of neural progenitors (By similarity). Acts as a regulator of the unfolded protein response (UPR) (PubMed:28915629). Belongs to the YOS1 family.