Phospholipid scramblase involved in lipid homeostasis and membrane dynamics processes. Has phospholipid scramblase activity toward cholesterol and phosphatidylserine, as well as phosphatidylethanolamine and phosphatidylcholine. Required for autophagosome formation: participates in early stages of autophagosome biogenesis at the endoplasmic reticulum (ER) membrane by reequilibrating the leaflets of the ER as lipids are extracted by atg2 (atg2a or atg2b) to mediate autophagosome assembly. In addition to autophagy, involved in other processes in which phospholipid scramblase activity is required. Modulates ER contacts with lipid droplets, mitochondria and endosomes. a 1,2-diacyl-sn-glycero-3-phospho-L-serine(in) = a 1,2-diacyl-sn-glycero-3-phospho-L-serine(out) cholesterol(in) = cholesterol(out) a 1,2-diacyl-sn-glycero-3-phosphocholine(in) = a 1,2-diacyl-sn-glycero-3-phosphocholine(out) a 1,2-diacyl-sn-glycero-3-phosphoethanolamine(in) = a 1,2-diacyl-sn-glycero-3-phosphoethanolamine(out) The VTT domain was previously called the SNARE-assoc domain. As there is no evidence that this domain associates with SNARE proteins, it was renamed as VMP1, TMEM41, and TVP38 (VTT) domain. Belongs to the VMP1 family.