The lens membrane skeleton contains structures preferentially enriched in spectrin-actin or tropomodulin-actin complexes

The spectrin-based membrane skeleton plays an important role in determining the distributions and densities of receptors, ion channels, and pumps, thu s influencing cell shape and deformability, cell polarity, and adhesion. In the paradigmatic human erythrocyte, short tropomodulin-capped actin filame nts are cross-linked by spectrin into a hexagonal network, yet the extent t o which this type of actin filament organization is utilized in the membran e skeletons of nonerythroid cells is not known. Here, we show that associat ions of tropomodulin and spectrin with actin in bovine lens fiber cells are distinct from that of the erythrocyte and imply a very different molecular organization. Mechanical disruption of the lens fiber cell membrane skelet on releases tropomodulin and actin-containing oligomeric complexes that can be isolated by gel filtration column chromatography, sucrose gradient cent rifugation and immunoadsorption. These tropomodulin-actin complexes do not contain spectrin. Instead, spectrin is associated with actin in different c omplexes that do not contain tropomodulin. Immunofluorescence staining of i solated fiber cells further demonstrates that tropomodulin does not precise ly colocalize with spectrin along the lateral membranes of lens fiber cells . Taken together, our data suggest that tropomodulin-capped actin filaments and spectrin-cross-linked actin filaments are assembled in distinct struct ures in the lens fiber cell membrane skeleton, indicating that it is organi zed quite differently from that of the erythrocyte membrane skeleton. Cell Motil. Cytoskeleton 46:257-268, 2000. (C) 2000 Wiley-Liss, Inc.