EVOLUTION OF THE SPECTRIN REPEAT

We now know that the evolution of multidomain proteins has frequently involved genetic duplication events. These, however, are sometimes dif ficult to trace because of low sequence similarity between duplicated segments. Spectrin, the major component of the membrane skeleton that provides elasticity to the cell, contains tandemly repeated sequences of 106 amino acid residues. The same repeats are also present in alpha -actinin, dystrophin and utrophin. Sequence alignments and phylogeneti c trees of these domains allow us to interpret the evolutionary relati onship between these proteins, concluding that spectrin evolved from a lpha-actinin by an elongation process that included two duplications o f a block of seven repeats. This analysis shows how a modular protein unit can be used in the evolution of large cytoskeletal structures.