A BETA-SPECTRIN ISOFORM FROM DROSOPHILA (BETA-H) IS SIMILAR IN SIZE TO VERTEBRATE DYSTROPHIN

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 β subunits, is associated with diversity in cell shape and membrane architecture. Here we describe a spectrin isoform from Drosophila that consists of a conventional α spectrin subunit complexed with a novel high molecular weight β subunit (430 kD) that we term βH. The native αβH 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 βH subunit, βH is encoded by a different gene than the previously described Drosophila β-spectrin subunit but shows sequence similarity to β-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 β-spectrin isoform (βH) than to other members of the spectrin gene family such as α-spectrin and α-actinin.