α-Catenin binds directly to spectrin and facilitates spectrin-membrane assembly in vivo

The anchorage of spectrin to biological membranes is mediated by protein and phosphoinositol phospholipid interactions. In epithelial cells, a nascent spectrin skeleton assembles in regions of cadherin-mediated cell-cell contact, and conversely, cytoskeletal assembly is required to complete the cell-adhesion process. The molecular interactions guiding these processes remain incompletely understood. We have examined the interaction of spectrin with alpha -catenin, a component of the adhesion complex. Spectrin (alpha II beta II) and alpha -catenin co-precipitate from extracts of confluent Madin-Darby canine kidney, HT29, and Clone A cells and from solutions of purified spectrin and alpha -catenin in vitro, By surface plasmon resonance and in vitro binding assays, we find that alpha -catenin binds alpha II beta II spectrin with an apparent K-d of approximate to 20-100 nM. By gel-overlay assay, alpha -catenin binds recombinant beta II-spectrin peptides that include the first 313 residues of spectrin but not to peptides that lack this region. Similarly, the binding activity of alpha -catenin is fully accounted for in recombinant peptides encompassing the NH2-terminal 228 amino acid region of alpha -catenin. An in vivo role for the interaction of spectrin with alpha -catenin is suggested by the impaired membrane assembly of spectrin and its enhanced detergent solubility in Clone A cells that harbor a defective alpha -catenin. Transfection of these cells with wild-type alpha -catenin reestablishes alpha -catenin at the plasma membrane and coincidentally recruits spectrin to the membrane. We propose that ankyrin-independent interactions of modest affinity between alpha -catenin and the amino-terminal domain of beta -spectrin augment the interaction between alpha -catenin and actin, and together they provide a polyvalent linkage directing the topographic assembly of a nascent spectrin-actin skeleton to membrane regions enriched in E-cadherin.