Abstract
Spectrin is a major structural protein associated with the cytoplasmic surface of plasma membranes of many types of cells. To study the functions of spectrin, we transfected Caco-2 intestinal epithelial cells with a plasmid conferring neomycin resistance and encoding either actin-binding or ankyrin-binding domains of beta G-spectrin fused with beta-galactosidase. These polypeptides, in principle, could interfere with the interaction of spectrin with actin or ankyrin, as well as block normal assembly of alpha- and beta-spectrin subunits. Cells expressing the fusion proteins represented only a small fraction of neomycin-resistant cells, but they could be detected based on expression of beta-galactosidase. Cells expressing spectrin domains exhibited a progressive decrease in amounts of endogenous beta G-spectrin, although alpha-spectrin was still present. Beta G-spectrin-deficient cells lost epithelial cell morphology, became multinucleated, and eventually disappeared after 10-14 d in culture. Spectrin-associated membrane proteins, ankyrin and adducin, as well as the Na+,K(+)-ATPase, which binds to ankyrin, exhibited altered distributions in cells transfected with beta G-spectrin domains. E-cadherin and F-actin, in contrast to ankyrin, adducin, and the Na+,K(+)-ATPase, were expressed, and they exhibited unaltered distribution in beta G-spectrin-deficient cells. Cells transfected with the same plasmid encoding beta-galactosidase alone survived in culture as the major population of neomycin-resistant cells, and they exhibited no change in morphology or in the distribution of spectrin-associated membrane proteins. These results establish that beta G-spectrin is essential for the normal morphology of epithelial cells, as well as for their maintenance in monolayer culture.