Transduction of extracellular signals requires activation of membrane-bound receptors. The transmission of the external signal across the plasma membrane often leads to an increase in the intracellular level of calcium [1-3]. In neurons, stimulation can increase the level of calcium more than 1000-fold; from <0.1 pM to 200-300 μM [4], This internal signal triggers calcium-binding proteins such as calmodulin, calbindin or troponin C, usually by inducing conformation changes that allow interactions with specific target proteins. Whether the target itself is a key protein in a particular cellular process or a regulatory protein, the calcium- dependent interaction with the target will affect the cellular process, either by up- regulating or down-regulating it. One target for calcium signals is spectrin and the spectrin-based membrane skeleton. This membrane skeleton is necessary for proper red cell shape and plasticity as well as lipid asymmetry. In non-erythroid cells, the cellular role of spectrin is probably linked to its ability to connect integral membrane proteins with actin filaments [5-17]. Therefore spectrin may be involved in positioning molecules such as receptors, ion channels and cell adhesion molecules correctly in the plasma membrane [7,18-24]. Since spectrin interacts with several calcium-binding proteins as well as binds calcium directly, it seems very likely that calcium ions are important and even may control some of the properties of spectrin. In this review I will examine the calcium-binding properties of spectrin in detail and discuss possible implications for cellular functions.
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