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). {ECO:0000250|UniProtKB:Q6PDG8, ECO:0000269|PubMed:23084991}. This is the function of MON1A (MON1 vesicular trafficking associated A, Ensembl gene identifier ENSG00000164077).