Tethers the endoplasmic reticulum to the cell membrane and promotes the formation of appositions between the endoplasmic reticulum and the cell membrane. Binds glycerophospholipids in a barrel-like domain and may play a role in cellular lipid transport. Plays a role in FGF signaling via its role in the rapid internalization of FGFR1 that has been activated by FGF1 binding; this occurs most likely via the AP-2 complex. Promotes the localization of SACM1L at endoplasmic reticulum-plasma membrane contact sites (EPCS) (PubMed:27044890). Homodimer. Interacts with ESYT1 and ESYT3. Interacts with FGFR1 that has been activated by FGF1 binding. Interacts with the AP-2 complex; identified in a complex with the AP-2 complex and FGFR1. Localizes to endoplasmic reticulum-plasma membrane contact sites (EPCS) (PubMed:29469807, PubMed:23791178, PubMed:30220461, PubMed:27044890). Recruited to the cell membrane via the third C2 domain (PubMed:17360437). Widely expressed with high level in cerebellum. 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 (PubMed:24373768). 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; can bind two lipid molecules simultaneously. Binds a variety of lipids, including phosphatidylethanolamine, phosphatidylcholine and phosphatidylinositol (PubMed:24847877). Belongs to the extended synaptotagmin family. Truncated N-terminus.