Binding sites of calmodulin and actin on the brain spectrin, calspectin

We used rotary-shadowing electron microscopy to map the calmodulin-and actin-binding sites on the brain spectrin, calspectin (or fodrin). Calspectin dimers appeared as rods 110 nm long and joined in a head-to-head manner to form tetramers 220 nm long. We determined calmodulin-binding sites by a ferritin-labeling method combined with biotin-avidin complex formation. Ferritin particles were found to attach to the head parts of calspectin dimers at a position 10-20 nm from the top of the head. The number of the calmodulin-binding sites seemed to be only one for each dimer and two for each tetramer. In contrast, the actin-binding sites were localized at the tail ends of the calspectin molecules. The tetramers attached to muscle F-actin with their tail ends and often cross-linked adjacent filaments. The results are discussed in view of the analogy to the erythrocyte spectrin.