Crystal Structure of the N-terminal Region of Brain Spectrin Reveals A Helical Junction Region and A Stable Structural Domain

The crystal structure of a recombinant protein consisting of the first 147 residues of brain α spectrin was solved to 2.3 A. The N-terminal region consists of the partial domain (Helix C’) and the anti-parallel, triple helical coiled-coil first structural domain (helices A1, B1, and C1). The data revealed that each asymmetric unit contained two crystallographically independent structures (1 and 2). The crystal structure of the first structural domain resembled that of the first structural domain of erythroid α-spectrin, determined before by solution NMR studies, with some specific differences, especially at the N-terminal region, including Helix C’ and the region connecting Helix C’ with the first structural domain (the junction region). The first ten residues are in a disordered conformation, followed by Helix C’ with an apparent, flexible bend. The junction region exhibits a helical conformation in contrast with an unstructured junction region in erythroid α spectrin. A special feature that has not been reported in other spectrin domains is the long and flexible A1B1 loop of 13 residues. This loop is likely the recognition site for interaction with other proteins. Hydrogen bonds and hydrogen bond networks were identified in the first structural domain and compared with those in erythroid α-spectrin. We suggest that these hydrogen bonds might contribute toward the stability of brain and erythroid spectrin.