alpha-Catenin binds directly to spectrin and facilitates spectrin-membraneassembly in vivo

The anchorage of spectrin to biological membranes is mediated by protein an d phosphoinositol phospholipid interactions. In epithelial cells, a nascent spectrin skeleton assembles in regions of cadherin-mediated cell-cell cont act, and conversely, cytoskeletal assembly is required to complete the cell -adhesion process. The molecular interactions guiding these processes remai n incompletely understood. We have examined the interaction of spectrin wit h alpha -catenin, a component of the adhesion complex. Spectrin (alpha II b eta 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 sp ectrin with an apparent K-d of approximate to 20-100 nM. By gel-overlay ass ay, alpha -catenin binds recombinant beta II-spectrin peptides that include the first 313 residues of spectrin but not to peptides that lack this regi on. Similarly, the binding activity of alpha -catenin is fully accounted fo r in recombinant peptides encompassing the NH2-terminal 228 amino acid regi on of alpha -catenin. An in vivo role for the interaction of spectrin with alpha -catenin is suggested by the impaired membrane assembly of spectrin a nd its enhanced detergent solubility in Clone A cells that harbor a defecti ve alpha -catenin. Transfection of these cells with wild-type alpha -cateni n reestablishes alpha -catenin at the plasma membrane and coincidentally re cruits spectrin to the membrane. We propose that ankyrin-independent intera ctions of modest affinity between alpha -catenin and the amino-terminal dom ain of beta -spectrin augment the interaction between alpha -catenin and ac tin, and together they provide a polyvalent linkage directing the topograph ic assembly of a nascent spectrin-actin skeleton to membrane regions enrich ed in E-cadherin.