Fig. 2. Schematic diagram of Arrow/LRP5/LRP6 and other members of the LDLR family. All proteins depicted are of human origin except for Arrow (Drosophila), and are conserved in vertebrates; some have invertebrate homologs (Herz and Bock, 2002). YWTD (Tyr, Trp, Thr and Asp)-type ß-propeller domains, LDLR type A (LA) domains and EGF (epidermal growth factor)-like domains are shared by all LDLR family proteins, although the domain arrangements vary. Arrow/LRP5/LRP6 are highly homologous and have the same extracellular domain arrangements. Intracellularly they do not have the NPxY [Asp, Pro, X (any amino acid) and Tyr] motif, but instead each have five copies of PPP(S/T)P (P, Pro; S/T, Ser or Thr) motifs. Besides Arrow/LRP5/LRP6, VLDLR (very low-density lipoprotein receptor) and APOER2 (apolipoprotein E receptor 2, also called LRP8) have established signaling roles by acting as receptors for the secreted signaling molecule Reelin. Other members of the LDLR family participate in lipoprotein/cholesterol uptake, steroid hormone uptake, regulation of cell surface protease activity and Ca²⁺ homeostasis, or are less characterized (reviewed by Herz and Bock, 2002). Many members have multiple names. The commonly used names are listed in bold.