1). Structural motifs of SED1/MFG-E8 and Del1.
Both proteins contain Notch like-EGF repeats, the second of which possess an RGD integrin-binding motif, as well as two discoidin/F5/8 C domains that are able to bind phospholipid bilayers and/or extracellular glycosides via 2-3 hairpin loops projecting from the central barrel core. SED1/MFG-E8 and Del1 are believed to facilitate cell-matrix adhesion via RGD-dependent binding to cell surface αvβ3/5 integrin receptors, whereas the discoidin/F5/8C domain is also thought to bind to cell membranes via intercalation into the lipid bilayer. Alternatively, the discoidin/F5/8C domains may coordinate binding to extracellular glycoside substrates, similar to that seen in the sugar-binding discoidin domains. Del1 contains a third EGF repeat not found within SED1.