Plays an important role in membrane trafficking through the secretory apparatus. Not involved in endocytic trafficking to lysosomes (PubMed:17632513). 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 (By similarity). Interacts with CCZ1 (By similarity). Found in a complex with RMC1, CCZ1, MON1A and MON1B (By similarity). The MON1A-CCZ1B complex interacts with RIMOC1 (By similarity). The MON1A-CCZ1B complex interacts with RAB7A and this interaction is enhanced in the presence of RIMOC1 (By similarity). Widely expressed (at protein level). The variant Ser-374 is unique to C57BL strains. It may confer the low iron phenotype observed in these strains. Belongs to the MON1/SAND family.