Guanine nucleotide exchange factor (GEF) which may activate RAB9A and RAB9B. Promotes the exchange of GDP to GTP, converting inactive GDP-bound Rab proteins into their active GTP-bound form. In bronchial mucosa, mainly expressed in ciliated and basal epithelial cells and weakly in alveolar cells (at protein level). Tends to be down-regulated in lung cancers, immortalized bronchial epithelial cell lines and precancerous lesions.