Lipid-binding protein that transports phosphatidylinositol, the precursor of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), from its site of synthesis in the endoplasmic reticulum to the cell membrane (By similarity). It thereby maintains the pool of cell membrane phosphoinositides, which are degraded during phospholipase C (PLC) signaling (By similarity). Plays a key role in the coordination of Ca(2+) and phosphoinositide signaling: localizes to sites of contact between the endoplasmic reticulum and the cell membrane, where it tethers the two bilayers (By similarity). In response to elevation of cytosolic Ca(2+), it is phosphorylated at its C-terminus and dissociates from the cell membrane, abolishing phosphatidylinositol transport to the cell membrane (By similarity). Positively regulates insulin secretion in response to glucose (PubMed:24012759). Phosphatidylinositol transfer to the cell membrane allows replenishment of PI(4,5)P2 pools and calcium channel opening, priming a new population of insulin granules (By similarity). Homodimer. Localizes to sites of contact between the endoplasmic reticulum and the cell membrane. Embedded into the endoplasmic reticulum membrane via its N-terminal transmembrane domain and associates with cell membrane via its C-terminus. In response to elevation of cytosolic Ca(2+), it is phosphorylated at its C-terminus and dissociates from the cell membrane and localizes to the reticular endoplasmic reticulum. Reassociates with cell membrane upon dephosphorylation. The SMP-LBD domain is a lipid transport module, which binds glycerolipids with a preference for phosphatidylinositol (PI). Phosphorylation at the C-terminus acidifies the protein and leads to disassociation from the acidic cell membrane. Reassociates with the cell membrane upon dephosphorylation.