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) (By similarity). 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).