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 α-catenin, a component of the adhesion complex. Spectrin (αIIβII) and α-catenin coprecipitate from extracts of confluent Madin-Darby canine kidney, HT29, and Clone A cells and from solutions of purified spectrin and α-catenin in vitro . By surface plasmon resonance and in vitro binding assays, we find that α-catenin binds αIIβII spectrin with an apparent K d of ≈20–100 n m . By gel-overlay assay, α-catenin binds recombinant βII-spectrin peptides that include the first 313 residues of spectrin but not to peptides that lack this region. Similarly, the binding activity of α-catenin is fully accounted for in recombinant peptides encompassing the NH 2 -terminal 228 amino acid region of α-catenin. An in vivo role for the interaction of spectrin with α-catenin is suggested by the impaired membrane assembly of spectrin and its enhanced detergent solubility in Clone A cells that harbor a defective α-catenin. Transfection of these cells with wild-type α-catenin reestablishes α-catenin at the plasma membrane and coincidentally recruits spectrin to the membrane. We propose that ankyrin-independent interactions of modest affinity between α-catenin and the amino-terminal domain of β-spectrin augment the interaction between α-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.