Involved in protein trafficking vesicle formation, probably by stabilizing of coatomer at the Golgi membrane and thus allowing the efficient formation of COPI coated vesicles. Interacts with MST28. Binds to coatomer proteins of COPI and SEC23/SEC24 of COPII coated vesicles. Members of the DUP240 multigene family are specific to S.cerevisiae sensu strictu. Cells lacking all 10 DUP240 proteins show no obvious alterations in mating, sporulation and cell growth. Belongs to the DUP/COS family.