The function of Ensembl gene identifier ENSG00000179331 (RAB39A, RAB39A, member RAS oncogene family) is as follows. The small GTPases Rab are key regulators of intracellular membrane trafficking, from the formation of transport vesicles to their fusion with membranes (PubMed:21255211). Rabs cycle between an inactive GDP-bound form and an active GTP-bound form that is able to recruit to membranes different sets of downstream effectors directly responsible for vesicle formation, movement, tethering and fusion (PubMed:21255211). RAB39A regulates autophagosome-lysosome fusion via recruitment of the HOPS endosomal tethering complex onto lysosomes; this process involves lysosomal RAB39A and autophagosomal RAB2A recruitment of HOPS subcomplexes VPS41-VPS16-VPS18-VPS33A and VPS39- VPS11, respectively, which assemble into a functional complex to mediate membrane tethering and SNAREs-driven membrane fusion (PubMed:37821429). Also negatively regulates lipopolysaccharide (LPS)- induced autophagosome formation in macrophages, possibly by implicating PI3K (PubMed:24349490). Promotes the delivery of MHC-I molecules from the ER to phagosomes and the generation of peptide-loaded MHC-I complexes in phagosomes, thus enhancing antigen cross-presentation by dendritic cells (By similarity). Plays a role in the maturation and acidification of phagosomes that engulf pathogens, such as S.aureus and M.tuberculosis (PubMed:21255211). Plays a role in the fusion of phagosomes with lysosomes (PubMed:21255211). May be involved in multiple neurite formation (By similarity). {ECO:0000250|UniProtKB:Q8BHD0, ECO:0000269|PubMed:21255211, ECO:0000269|PubMed:24349490, ECO:0000269|PubMed:37821429}.