We report the crystal structure of two adjacent fibronectin type III repeats from the Drosophila neural cell adhesion molecule neuroglian. Each domain consists of two antiparallel 0 sheets and is folded topologically identically to single fibronectin type III domains from the extracellular matrix proteins tenascin and fibronectin. β bulges and left-handed polyproline II helices disrupt the regular β sheet structure of both neuroglian domains. The hydrophobic interdomain interface includes a metalbinding site, presumably involved in stabilizing the relative orientation between domains and predicted by sequence comparision to be present in the vertebrate homolog molecule L1. The neuroglian domains are related by a near perfect 2-fold screw axis along the longest molecular dimension. Using this relationship, a model for arrays of tandem fibronectin type III repeats in neuroglian and other molecules is proposed.
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