Acts in concert with MON1A, 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 MON1A (PubMed:23084991). Found in a complex with RMC1, CCZ1, MON1A and MON1B (PubMed:29038162). Belongs to the CCZ1 family.