Plays an important role in membrane trafficking through the secretory apparatus. Not involved in endocytic trafficking to lysosomes (By similarity). Acts in concert with CCZ1, as a guanine exchange factor (GEF) for RAB7, promotes the exchange of GDP to GTP, converting it from an inactive GDP-bound form into an active GTP-bound form (PubMed:23084991). Interacts with CCZ1 (PubMed:23084991). Found in a complex with RMC1, CCZ1, MON1A and MON1B (PubMed:29038162). The MON1A-CCZ1B complex interacts with RIMOC1 (PubMed:34432599). The MON1A-CCZ1B complex interacts with RAB7A and this interaction is enhanced in the presence of RIMOC1 (PubMed:34432599). Belongs to the MON1/SAND family. Truncated N-terminus.