Required for fusion of ER-derived vesicles with the Golgi during ER-to-Golgi protein transport. May be involved in proper membrane localization of Rab GTPases. Component of the YIP1-YIF1 complex, composed of at least YIF1, YIP1 and YOS1. The complex interacts with the ER to Golgi SNAREs BOS1 and SEC22. Interacts with the YIP1 family members YIP4 and YIP5, and with the Rab GTPases SEC4, YPT1, YPT6, YPT7, YPT10, YPT11, YPT31, YPT32 and YPT52. Interacts with BTN2 and SNX3. Also found in ER-derived COPII-coated vesicles. Belongs to the YIF1 family.