Ankyrin-G and beta2-spectrin collaborate in biogenesis of lateral membrane of human bronchial epithelial cells

Ankyrins are a family of adapter proteins required for localization of membrane proteins to diverse specialized membrane domains including axon initial segments, specialized sites at the transverse tubule/sarcoplasmic reticulum in cardiomyocytes, and lateral membrane domains of epithelial cells. Little is currently known regarding the molecular basis for specific roles of different ankyrin isoforms. In this study, we systematically generated alanine mutants of clusters of charged residues in the spectrin-binding domains of both ankyrin-B and -G. The corresponding mutants were evaluated for activity in either restoration of abnormal localization of the inositol trisphosphate receptor in the sarcoplasmic reticulum in mutant mouse cardiomyocytes deficient in ankyrin-B or in prevention of loss of lateral membrane in human bronchial epithelial cells depleted of ankyrin-G by small interfering RNA. Interestingly, ankyrin-B and -G share two homologous sites that result in loss of function in both systems, suggesting that common molecular interactions underlie diverse roles of these isoforms. Ankyrins G and B also exhibit differences; mutations affecting spectrin binding had no effect on ankyrin-B function but did abolish activity of ankyrin-G in restoring lateral membrane biogenesis. Depletion of beta(2)-spectrin by small interfering RNA phenocopied depletion of ankyrin-G and resulted in a failure to form new lateral membrane in interphase and mitotic cells. These results demonstrate that ankyrin-G and beta(2)-spectrin are functional partners in biogenesis of the lateral membrane of epithelial cells.