Guanine nucleotide exchange factor (GEF) regulating homeostasis of late endocytic pathway, including endosomal positioning, maturation and secretion, possibly through activating Rab proteins such as RAB27A and RAB27B. Seems to promote the exchange of GDP to GTP, converting inactive GDP-bound RAB27A and RAB27B into their active GTP-bound form. Interacts with RAB27A and RAB27B (GDP-bound forms preferentially). Belongs to the DENND10 family.