The Caenorhabditis elegans genome encodes one α spectrin subunit, a β spectrin subunit (β-G), and a β-H spectrin subunit. Our experiments show that the phenotype resulting from the loss of the C. elegans α spectrin is reproduced by tandem depletion of both β-G and β-H spectrins. We propose that α spectrin combines with the β-G and β-H subunits to form α/β-G and α/β-H heteromers that perform the entire repertoire of spectrin function in the nematode. The expression patterns of nematode β-G spectrin and vertebrate β spectrins exhibit three striking parallels including: (1) β spectrins are associated with the sites of cell–cell contact in epithelial tissues; (2) the highest levels of β-G spectrin occur in the nervous system; and (3) β spec-trin-G in striated muscle is associated with points of attachment of the myofilament apparatus to adjacent cells. Nematode β-G spectrin associates with plasma membranes at sites of cell–cell contact, beginning at the two-cell stage, and with a dramatic increase in intensity after gastrulation when most cell proliferation has been completed. Strikingly, depletion of nematode β-G spectrin by RNA-mediated interference to undetectable levels does not affect the establishment of structural and functional polarity in epidermis and intestine. Contrary to recent speculation, β-G spectrin is not associated with internal membranes and depletion of β-G spectrin was not associated with any detectable defects in secretion. Instead β-G spectrin-deficient nematodes arrest as early larvae with progressive defects in the musculature and nervous system. Therefore, C. elegans β-G spectrin is required for normal muscle and neuron function, but is dispensable for embryonic elongation and establishment of early epithelial polarity. We hypothesize that heteromeric spectrin evolved in metazoans in response to the needs of cells in the context of mechanically integrated tissues that can withstand the rigors imposed by an active organism.

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~-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 betaspectrin 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 betac-spectrin, although alpha-spectrin was still present. Betac-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 betac-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 betao-spectrin-deficient cells. Cells transfected with the same plasmid encoding betagalactosidase 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 betac-spectrin is essential for the normal morphology of epithelial cells, as well as for their maintenance in monolayer culture. S PV.CTRIN is an elongated actin-binding protein that is the principal component of a system of structural proteins associated with the cytoplasmic surface of plasma membranes of most metazoan cells (reviewed by Bennett and Gilligan, 1993). Spectrin is comprised of two subunits, termed alpha and beta, that are aligned side-to-side to form heterodimers, and the dimers are linked head-to-head to form tetramers. Beta subunits contain most of the recognition sites of spectrin for other proteins including ankyrin, protein 4.1 actin, as well as the site for ankyrin-independent association of spectrin with membranes. Beta~-spectrin is the most common type of beta subunit, and it is expressed in most vertebrate tissues (Hu et al., 1992). The structure and function of spectrin has been best characterized in mammalian erythrocytes from both in vivo and in vitro studies (Palek and Lambert, 1990;Delaunay and Dhermy, 1993;Gallagher and Forget, 1993;Bennett and Gilligan, 1993). Erythrocyte spectrin forms a membrane-associated polygonal network by the associations of spectrin with actin illaments and with integral membrane proteins through linkages with ankyrin and protein 4.1. Defects and deficiencies in components of the spectrin-actin network result in abnormally fr...