The self-association domain of spectrin can be modeled as a triple helix.

(Top) Alignment of the primary sequence of Drosophilaα-spectrin 14th unit with the partial repeat units from the amino-terminus of human αΙ-spectrin (residues 19-51) and from the 17th repeat unit of βΙ-spectrin (residues 2008-2080), using the program Bestfit.35 The position of the 3 helices as determined from the crystal structure of the Drosophilaspectrin are shown. Continuity was not assumed where the 2 subunits join in the concatenated sequence. This concatenated sequence, with or without the mutations described in the text, was then treated as a pseudo-106 residue repeat unit for the dynamic modeling studies. Dros indicates Drosophila; term, terminus. (Bottom) Comparison of the calculated structure of Drosophila α-spectrin 14th repeat (blue) or the concatenated human αΙβΙ self-association domain (green) with the crystal structure of the Drosophila14th repeat (red). Two views are shown, longitudinal on the left, and an end-on view on the right. The 6-Å hydration shell involving 1100 water molecules is not shown. This hydration shell is required for the predicted structure to fold to a triple-helical unit. Note the close correspondence of the fitted structures with the crystal, even though in the case of the self-association domain, 70% of the residues differ from those in the Drosophila protein.