Binds glycerophospholipids in a barrel-like domain and may play a role in cellular lipid transport (By similarity). Tethers the endoplasmic reticulum to the cell membrane and promotes the formation of appositions between the endoplasmic reticulum and the cell membrane. Interacts with ESYT1 and ESYT2. Localizes to endoplasmic reticulum-plasma membrane contact sites (EPCS) (PubMed:29469807, PubMed:30220461). Recruited to the cell membrane via the third C2 domain. Widely expressed with high level in cerebellum and skin. Anchored to the endoplasmic reticulum membrane by a transmembrane hairpin structure; both N-terminus and C-terminus are cytoplasmic. The C2 domains mediate lipid and calcium binding. The N-terminal C2 domain binds calcium ions and is important for calcium-dependent lipid binding and interaction with membranes. Two calcium ions are bound at a high-affinity site and a third calcium ion is bound with lower affinity. May bind up to four calcium ions. In contrast, the second C2 domain apparently does not bind calcium (By similarity). The third C2 domain mediates interaction with membranes enriched in phosphatidylinositol 4,5-bisphosphate and is required for location at the cell membrane (PubMed:23791178). The SMP-LTD domain is a barrel-like domain that binds glycerophospholipids in its interior (By similarity). Belongs to the extended synaptotagmin family.