Guanine nucleotide exchange factor that specifically activates the small GTPase SAR1B. Mediates the recruitment of SAR1B and other COPII coat components to endoplasmic reticulum membranes and is therefore required for the formation of COPII transport vesicles from the ER. Was first identified based on its probable role in the regulation of pituitary gene transcription. Binds to the prolactin gene (PRL) promoter and seems to activate transcription. Interacts with SAR1B (GDP-bound form) (PubMed:11422940). Interacts with MIA2; recruits PREB to endoplasmic reticulum exit sites (By similarity). Concentrates at endoplasmic reticulum exit sites (ERES), also known as transitional endoplasmic reticulum (tER). Detected starting at 10.5 dpc in the ectoderm and superficial mesoderm surrounding the caudal part of the tail region. At 12.5 dpc detected in dorsal root ganglia, surface ectoderm surrounding the caudal part of the tail, the inferior wall of the genital tubercle, developing liver, in Rathke pouch and in condensing mesenchyme forming the roof of the skull. At 14.5 dpc detected in the perichondrial region of the craniofacial, axial, and appendicular skeleton. By 16.5 dpc expression is much lower throughout the embryo, and is not detectable by 18.5 dpc.