Required for proper maturation of seed storage proteins. Forms a complex with MAG2, ZW10/MIP1 and MIP3 on the endoplasmic reticulum that may be responsible for efficient transport of seed storage proteins. Forms a complex with MAG2, ZW10/MIP1 and MIP3 on the endoplasmic reticulum. A number of isoforms are produced. According to EST sequences. Accumulation of the precursors of the two major storage proteins albumin 2S and globulin 12S in dry seeds.