This is a lipid transport domain found in phospholipid transfer proteins such as C2CD2L-like (also known as TMEM24). The TMEM24-SMP domain is shown to bind glycerolipids with a preference for phosphatidylinositol (PI).The bound PI is then transferred to the plasma membrane (PM) where it is converted to phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] to replenish pools of this lipid hydrolyzed during glucose-stimulated signaling. PI(4,5)P2 is required for Ca2+-dependent exocytosis hence, the SMP domain of TMEM24 is essential for sustaining the intracellular Ca2+ oscillations that trigger bursts of insulin granule release and hence insulin secretion. The SMP domain belongs to a superfamily of lipid/hydrophobic ligand-binding domains called TULIP for (tubular lipid-binding proteins) it adopts TULIP fold with two alpha helices and a highly curved antiparallel beta sheet forming a cornucopia-like structure [1].

1: Lipid transport by TMEM24 at ER-plasma membrane contacts regulates pulsatile insulin secretion. Lees JA, Messa M, Sun EW, Wheeler H, Torta F, Wenk MR, De Camilli P, Reinisch KM; Science. 2017; [Epub ahead of print] PMID:28209843