A genetic screen to uncover mechanisms underlying lipid transfer protein function at membrane contact sites

Lipid transfer proteins mediate the transfer of lipids between organelle membranes, and the loss of function of these proteins has been linked to neurodegeneration. However, the mechanism by which loss of lipid transfer activity leads to neurodegeneration is not understood. In Drosophila photoreceptors, depletion of retinal degeneration B (RDGB), a phosphatidylinositol transfer protein, leads to defective phototransduction and retinal degeneration, but the mechanism by which loss of this activity leads to retinal degeneration is not understood. RDGB is localized to membrane contact sites through the interaction of its FFAT motif with the ER integral protein VAP. To identify regulators of RDGB function in vivo, we depleted more than 300 VAP-interacting proteins and identified a set of 52 suppressors of rdgB The molecular identity of these suppressors indicates a role of novel lipids in regulating RDGB function and of transcriptional and ubiquitination processes in mediating retinal degeneration in rdgB9 The human homologs of several of these molecules have been implicated in neurodevelopmental diseases underscoring the importance of VAP-mediated processes in these disorders.

Structural organization of RDGB (retinal degeneration B), a multi-domain lipid transfer protein: a molecular modelling and simulation based approach

Lipid transfer proteins (LTPs) that shuttle lipids at membrane contact sites (MCS) play an important role in maintaining cellular homeostasis. One such important LTP is the Retinal Degeneration B (RDGB) protein. RDGB is localized at the MCS formed between the endoplasmic reticulum (ER) and the apical plasma membrane (PM) in Drosophila photoreceptors where it transfers phosphatidylinositol (PI) during G-protein coupled phospholipase C signalling. Previously, the C-terminal domains of RDGB have been shown to be essential for its function and accurate localization. In this study, using in-silico integrative modelling we predict the structure of entire RDGB protein in complex with the ER membrane protein VAP. The structure of RDGB has then been used to decipher the structural features of the protein important for its orientation at the contact site. Using this structure, we identify two lysine residues in the C-terminal helix of the LNS2 domain important for interaction with the PM. Using molecular docking, we also identify an unstructured region USR1, immediately c-terminal to the PITP domain that is important for the interaction of RDGB with VAP. Overall the 10.06 nm length of the predicted RDGB-VAP complex spans the distance between the PM and ER and is consistent with the cytoplasmic gap between the ER and PM measured by transmission electron microscopy in photoreceptors. Overall our model explains the topology of the RDGB-VAP complex at this ER-PM contact site and paves the way for analysis of lipid transfer function in this setting.Communicated by Ramaswamy H. Sarma.

Extended synaptotagmin regulates membrane contact site structure and lipid transfer function in vivo

Inter-organelle communication between closely apposed membranes is proposed at membrane contact sites (MCS). However, the regulation of MCS structure and their functional relevance in vivo remain debated. The extended synaptotagmins (Esyt) are evolutionarily conserved proteins proposed to function at MCS. However, loss of all three Esyts in yeast or mammals shows minimal phenotypes questioning the functional importance of Esyt. We report that in Drosophila photoreceptors, MCS number is regulated by PLCβ activity. Photoreceptors of a null allele of Drosophila extended synaptotagmin (dEsyt) show loss of ER-PM MCS. Loss of dEsyt results in mislocalization of RDGB, an MCS localized lipid transfer protein, required for photoreceptor structure and function, ultimately leading to retinal degeneration. dEsyt depletion enhanced the retinal degeneration, reduced light responses and slower rates of plasma membrane PIP₂ resynthesis seen in rdgB mutants. Thus, dEsyt function and PLCβ signaling regulate ER-PM MCS structure and lipid transfer in Drosophila photoreceptors.