Involved in the selective transport and maturation of TGF-alpha family proteins. Interacts with AREG immature precursor and with immature TGFA, i.e. with a prosegment and lacking full N-glycosylation, but not with the fully N-glycosylated form. In the Golgi apparatus, may form a complex with GORASP55 and transmembrane TGFA (By similarity). Located primarily in the ER; may cycle between the ER and the Golgi apparatus. Belongs to the cornichon family.