Structural studies of the shortest extended synaptotagmin with only two C2 domains from Trypanosoma brucei

Abstract

Summary Extended synaptotagmins (E-Syts) localize at membrane contact sites between the endoplasmic reticulum (ER) and the plasma membrane to mediate inter-membrane lipid transfer and control plasma membrane lipid homeostasis. All known E-Syts contain an N-terminal transmembrane (TM) hairpin, a central synaptotagmin-like mitochondrial lipid-binding protein (SMP) domain, and three or five C2 domains at their C termini. Here we report an uncharacterized E-Syt from the protist parasite Trypanosoma brucei , namely, TbE-Syt. TbE-Syt contains only two C2 domains (C2A and C2B), making it the shortest E-Syt known by now. We determined a 1.5-Å-resolution crystal structure of TbE-Syt-C2B and revealed that it binds lipids via both Ca 2+ - and PI(4,5)P 2 -dependent means. In contrast, TbE-Syt-C2A lacks the Ca 2+ -binding site but may still interact with lipids via a basic surface patch. Our studies suggest a mechanism for how TbE-Syt tethers the ER membrane tightly to the plasma membrane to transfer lipids between the two organelles.

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“…Like to all mammalian cells, E-Syts from T. brucei has an N-terminal transmembrane hairpin and a central synaptotagmin-like mitochondrial lipid binding protein (SMP). However, TbE-Syt contains only two C2 domains, named C2A and C2B in contrast to at least three C2 domains in all other reported E-Syts present in higher eukaryotic cells ( SahekiDe Camilli, 2017 ; Stepinac et al., 2021 ). The depletion of TbE-Syt had no significant effect in bloodstream form T. brucei but have deleterious effect in procyclic forms.…”

“…The depletion of TbE-Syt had no significant effect in bloodstream form T. brucei but have deleterious effect in procyclic forms. Immunocytochemistry analysis showed that TbE-Sys co-localizes to both BiP, at the central ER close to nuclear envelope, and (FAZ1) a marker protein of unique flagellar attachment zone (FAZ) – associated cortical ER ( Stepinac et al., 2021 ). The FAZ is a large macromolecular structure located at the interface formed by the cellular and the flagellar membranes.…”

“…The TbE-Syt might thus anchor and staple the FAZ-associated cortical ER to the plasma membrane, allowing the dimeric SMP domains, to transport lipid molecules in their hydrophobic clefts, from ER to the plasma membrane. ( Stepinac et al., 2021 ).…”