Binds glycerophospholipids in a barrel-like domain and may play a role in cellular lipid transport (By similarity). Binds calcium (via the C2 domains) and translocates to sites of contact between the endoplasmic reticulum and the cell membrane in response to increased cytosolic calcium levels. Helps tether the endoplasmic reticulum to the cell membrane and promotes the formation of appositions between the endoplasmic reticulum and the cell membrane (By similarity). Interacts with ESYT2 and ESYT3. Interacts (phosphorylated form) with SLC2A4 (By similarity). Localizes primarily to the endoplasmic reticulum. Recruited to sites of contact between the endoplasmic reticulum and the cell membrane in response to increased cytosolic calcium levels. 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. The third C2 domain mediates interaction with membranes enriched in phosphatidylinositol 4,5-bisphosphate and is required for translocation to the cell membrane in response to increased cytosolic calcium levels (By similarity). The SMP-LTD domain is a barrel-like domain that binds glycerophospholipids in its interior (By similarity). Phosphorylated on Ser residues in insulin-treated adipocytes (in vitro); this promotes interaction with SLC2A4. Belongs to the extended synaptotagmin family.