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Reelin, a large secreted protein implicated in the cortical development of the mammalian brain, is composed of eight tandem concatenations of 'reelin repeats' and binds to neuronal receptors belonging to the low-density lipoprotein receptor gene family. We found that both receptor-binding and subsequent Dab1 phosphorylation occur solely in the segment spanning the fifth and sixth reelin repeats (R5-6). Monomeric fragment exhibited a suboptimal level of signaling activity and artificial oligomerization resulted in a 10-fold increase in activity, indicating the critical importance of higher-order multimerization in physiological reelin. A 2.0-A crystal structure from the R5-6 fragment revealed not only a unique domain arrangement wherein two repeats were aligned side by side with the same orientation, but also the unexpected presence of bound Zn ions. Structure-guided alanine mutagenesis of R5-6 revealed that two Lys residues (Lys-2360 and Lys-2467) constitute a central binding site for the low-density lipoprotein receptor class A module in the receptor, indicating a strong similarity to the ligand recognition mode shared among the endocytic lipoprotein receptors. brain development | x-ray crystallography