Required for proper maturation of seed storage proteins. Forms a complex with MAG2, ZW10/MIP1 and MIP2 on the endoplasmic reticulum that may be responsible for efficient transport of seed storage proteins. Forms a complex with MAG2, ZW10/MIP1 and MIP2 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. Belongs to the STXBP/unc-18/SEC1 family.