8.A.97. The EPG-3/VMP1 (EPG/VPM) Scramblase Family
During autophagosome formation in mammalian cells, isolated membranes (IM) (autophagosome precursors) dynamically contact the ER. Zhao et al. 2017 demonstrated that the ER-localized metazoan-specific autophagy protein EPG-3/VMP1 controls ER-IM contacts. Loss of VMP1 causes stable association of the IM with the ER, thus blocking autophagosome formation. Interaction of WIPI2 with the ULK1/FIP200 complex and PI3P contributes to the formation of ER-IM contacts, and these interactions are enhanced by VMP1 depletion. VMP1 controls contact formation by promoting SERCA (sarco[endo]plasmic reticulum calcium ATPase) activity. VMP1 interacts with SERCA and prevents formation of the SERCA/PLN/SLN inhibitory complex. VMP1 also modulates ER contacts with lipid droplets, mitochondria, and endosomes. These ER contacts are elevated by the SERCA inhibitor thapsigargin. Calmodulin acts as a sensor/effector to modulate the ER contacts mediated by VMP1/SERCA. Zhao et al. 2017 thus reveals that VMP1 modulates SERCA activity to control ER contacts.
Human VMP1 is a stress-induced protein that, when
overexpressed, promotes formation of intracellular vacuoles followed by
cell death. It may be involved in the cytoplasmic vacuolization of acinar
cells during the early stage of acute pancreatitis, and it plays a role in the
initial stages of the autophagic process through its interaction with
BECN1. It may also be involved in cell-cell adhesion and plays an
essential role in formation of cell junctions (Sauermann et al. 2008). The ectopic P-granules protein, EPG-3, of C. elegans is also thought to act in autophagasome and omegasome formation (Tian et al. 2010). These proteins appear to contain a DedA domain.