title: Extended synaptotagmins are Ca2+-dependent lipid transfer proteins at membrane contact sites.
abstract: Organelles are in constant communication with each other through exchange of proteins (mediated by trafficking vesicles) and lipids [mediated by both trafficking vesicles and lipid transfer proteins (LTPs)]. It has long been known that vesicle trafficking can be tightly regulated by the second messenger Ca(2+), allowing membrane protein transport to be adjusted according to physiological demands. However, it remains unclear whether LTP-mediated lipid transport can also be regulated by Ca(2+) In this work, we show that extended synaptotagmins (E-Syts), poorly understood membrane proteins at endoplasmic reticulum-plasma membrane contact sites, are Ca(2+)-dependent LTPs. Using both recombinant and endogenous mammalian proteins, we discovered that E-Syts transfer glycerophospholipids between membrane bilayers in the presence of Ca(2+) E-Syts use their lipid-accommodating synaptotagmin-like mitochondrial lipid binding protein (SMP) domains to transfer lipids. However, the SMP domains themselves cannot transport lipids unless the two membranes are tightly tethered by Ca(2+)-bound C2 domains. Strikingly, the Ca(2+)-regulated lipid transfer activity of E-Syts was fully recapitulated when the SMP domain was fused to the cytosolic domain of synaptotagmin-1, the Ca(2+)sensor in synaptic vesicle fusion, indicating that a common mechanism of membrane tethering governs the Ca(2+)regulation of lipid transfer and vesicle fusion. Finally, we showed that microsomal vesicles isolated from mammalian cells contained robust Ca(2+)-dependent lipid transfer activities, which were mediated by E-Syts. These findings established E-Syts as a novel class of LTPs and showed that LTP-mediated lipid trafficking, like vesicular transport, can be subject to tight Ca(2+)regulation.
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of Colorado at Boulder, Boulder, CO 80309.
authors.forename: Haijia
authors.initials: H
authors.lastname: Yu
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of Colorado at Boulder, Boulder, CO 80309.
authors.forename: Yinghui
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of Colorado at Boulder, Boulder, CO 80309.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Gulbranson
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of Colorado at Boulder, Boulder, CO 80309.
authors.forename: Alex
authors.initials: A
authors.lastname: Paine
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of Colorado at Boulder, Boulder, CO 80309.
authors.forename: Shailendra S
authors.initials: SS
authors.lastname: Rathore
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of Colorado at Boulder, Boulder, CO 80309 jingshi.shen@colorado.edu.
authors.forename: Jingshi
authors.initials: J
authors.lastname: Shen
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000502:physiology; D002118:Calcium / Q000737:chemistry / Q000378:metabolism*; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism*; D050356:Lipid Metabolism / Q000502:physiology*; D050857:Synaptotagmins / Q000737:chemistry / Q000378:metabolism*
chemical_list: D050857:Synaptotagmins; D002118:Calcium
keywords: lipid transfer; membrane contact sites; organelle; synaptotagmin