Mutations in the human VPS13 genes are responsible for neurodevelopmental and neurodegenerative disorders including chorea acanthocytosis (VPS13A) and Parkinson's disease (VPS13C). The mechanisms of these diseases are unknown. Genetic studies in yeast hinted that Vps13 may have a role in lipid exchange between organelles. In this study, we show that the N-terminal portion of VPS13 is tubular, with a hydrophobic cavity that can solubilize and transport glycerolipids between membranes. We also show that human VPS13A and VPS13C bind to the ER, tethering it to mitochondria (VPS13A), to late endosome/lysosomes (VPS13C), and to lipid droplets (both VPS13A and VPS13C). These findings identify VPS13 as a lipid transporter between the ER and other organelles, implicating defects in membrane lipid homeostasis in neurological disorders resulting from their mutations. Sequence and secondary structure similarity between the N-terminal portions of Vps13 and other proteins such as the autophagy protein ATG2 suggest lipid transport roles for these proteins as well.

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Valverde et al. show that the autophagy protein ATG2 functions in autophagosome biogenesis by transferring lipids at ER–autophagosome contact sites.

INTRODUCTION Insulin is secreted by pancreatic β cells in response to glucose stimulation. Its release is controlled by the interplay of calcium and phosphoinositide signaling pathways. A rapid release phase, in which insulin containing granules that are already docked and primed at the plasma membrane (PM) undergo exocytosis, is followed by slow release. In this second phase, granules are docked and primed and then released in a series of bursts, each triggered by a spike in cytosolic Ca2+. RATIONALE To better understand the molecular basis underlying insulin secretion, we characterized TMEM24, a protein enriched in neuroendocrine cells previously suggested to be required for a normal secretory response. RESULTS We found that TMEM24 is an endoplasmic reticulum (ER) protein that concentrates at ER-PM contact sites, where it tethers the two bilayers. TMEM24 binding “in trans” to the PM is negatively regulated by phosphorylation in response to elevation of cytosolic Ca2+, so that TMEM24 transiently dissociates from the PM as Ca2+ concentration spikes and then reassociates with this membrane upon dephosphorylation. Additionally, TMEM24 contains a lipid transport module of the synaptotagmin-like, mitochondrial and lipid-binding protein (SMP) family, which we structurally characterized and showed to bind glycerolipids with a preference for phosphatidylinositol (PI). Thus, TMEM24 helps deliver PI, which is synthesized in the ER, to the PM, where it is converted to phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] to replenish pools of this lipid hydrolyzed during glucose-stimulated signaling. Supporting a key role of TMEM24 in the coordination of Ca2+ and phosphoinositide signaling, the lipid transport function of TMEM24 is essential for sustaining the intracellular Ca2+ oscillations that trigger bursts of insulin granule release and hence insulin secretion. PI(4,5)P2 is required for Ca2+-dependent exocytosis. It also controls the activity of PM ion channels that regulate cytosolic Ca2+ levels and is the precursor of IP3, which also helps to modulate cytosolic Ca2+ by triggering Ca2+ release from the ER. Thus, in insulin-secreting cells, TMEM24 participates in coordinating Ca2+ and phosphoinositide signaling pathways to cause pulsatile insulin secretion (see the figure). CONCLUSION Our findings implicate ER-PM contact sites and an ER resident lipid-transfer protein in the direct regulation of PM phosphoinositide pools, offering fresh insights into the mechanisms of cellular phosphoinositide dynamics. More specifically, they elaborate the mechanisms underlying insulin secretion, which is impaired in patients with type II diabetes, and may ultimately have therapeutic ramifications. TMEM24 activity cycle at ER-PM contacts Glucose stimulation of insulin-secreting cells triggers Ca2+ influx, phospholipase C–dependent PI(4,5)P2 cleavage, and granule exocytosis. Ca2+-stimulated phosphorylation causes TMEM24 dissociation from the PM and interruption of SMP-mediated PI transfer that allows PI(4,5)P2 resynthesis. Lower...

A single particle cryo-EM reconstruction of an ∼160-kD N-terminal fragment of the lipid transport protein VPS13 reveals an ∼160-Å long channel lined with hydrophobic residues suitable for solubilizing multiple lipid fatty acid moieties. The structure suggests that VPS13 and related proteins, like the autophagy protein ATG2, can act as bridges between organelle membranes to allow bulk lipid flow between organelles.