Spectrins are a major component of the membrane skeleton in many cell types where they are thought to contribute to cell form and membrane organization. Diversity among spectrin isoforms, especially their beta subunits, is associated with diversity in cell shape and membrane architecture. Here we describe a spectrin isoform from Drosophila that consists of a conventional alpha spectrin subunit complexed with a novel high molecular weight beta subunit (430 kD) that we term beta H. The native alpha beta H molecule binds actin filaments with high affinity and has a typical spectrin morphology except that it is longer than most other spectrin isoforms and includes two knoblike structures that are attributed to a unique domain of the beta H subunit. Beta H is encoded by a different gene than the previously described Drosophila beta-spectrin subunit but shows sequence similarity to beta-spectrin as well as vertebrate dystrophin, a component of the membrane skeleton in muscle. By size and sequence similarity, dystrophin is more similar to this newly described beta-spectrin isoform (beta H) than to other members of the spectrin gene family such as alpha-spectrin and alpha-actinin.

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