Lipid transfer and molecular tethering protein, which plays a key role in the formation of membrane contact sites between the endoplasmic reticulum and other organelles, such as late endosomes/lysosomes and mitochondria (PubMed:29097544, PubMed:31636202, PubMed:31818884, PubMed:32686675, PubMed:33912962, PubMed:40246839, PubMed:40839735, PubMed:41512824). Faciliates the exchange of lipids at membrane contact sites between the endoplasmic reticulum and organelles by mediating the transport of lipids, such as ceramide, cholesterol and phosphatidylserine (PubMed:33912962, PubMed:40839735). Also acts as a molecular glue by undergoing liquid-liquid phase separation, forming condensates that mediate stable adhesion between endoplasmic reticulum and surrounding organelles, connecting two distinct membrane-bound vesicles coupled together (PubMed:41512824). Connects the endoplasmic reticulum to late endosomes/lysosomes, facilitating the exchange of lipids, which is crucial for membrane remodeling and processes, such as neurite outgrowth (PubMed:31636202, PubMed:32686675, PubMed:33912962). PDZD8-dependent endoplasmic reticulum-mitochondria membrane tethering is essential for endoplasmic reticulum-mitochondria Ca(2+) transfer (PubMed:29097544, PubMed:31818884, PubMed:40246839). In neurons, involved in the regulation of dendritic Ca(2+) dynamics by regulating mitochondrial Ca(2+) uptake in neurons (PubMed:29097544). Mediates lysosomal vacuolation, a process during which lysosomes abnormally enlarge into large membrane-bound sacs in response to stress such as osmotic stress: acts by mediating bulk lipid transfer from the endoplasmic reticulum to lysosomes, leading to vacuole formation (PubMed:40839735). Plays an indirect role in the regulation of cell morphology and cytoskeletal organization (PubMed:21834987). May indirectly inhibit herpes simplex virus 1 infection at an early stage (PubMed:21549406). {ECO:0000269|PubMed:21549406, ECO:0000269|PubMed:21834987, ECO:0000269|PubMed:29097544, ECO:0000269|PubMed:31636202, ECO:0000269|PubMed:31818884, ECO:0000269|PubMed:32686675, ECO:0000269|PubMed:33912962, ECO:0000269|PubMed:40246839, ECO:0000269|PubMed:40839735, ECO:0000269|PubMed:41512824}. This is the function of ENSG00000165650 (PDZD8, PDZ domain containing 8).