Comparison of intestinal brush-border 95-kdalton polypeptide and alpha-actinins

To explore the suggestion that alpha-actinin cross-links actin filaments to the microvillar membrane (Mooseker and Tilney, 1975, J. Cell Biol. 67: 725-743; Mooseker, 1976, J. Cell Biol. 71: 417-433), we have assessed the possible relatedness of alpha-actinin and the brush-border 95-kdalton protein by four independent criteria: antigenicity, mobility on SDS gels, extractability in nonionic detergents, and peptide maps. We have found that anti-chicken gizzard alpha-actinin stains the junctional complex region of intact cells (Craig and Pardo, 1979, J. Cell Biol. 80: 203-210) but does not stain isolated brush borders even though these structures contain a 95-kdalton polypeptide. Lack of staining is not caused by failure of the antibody to penetrate, as antiactin stains both the terminal web and the microvilli of isolated brush borders. By the antibody SDS gel overlay technique, we have established that anti-gizzard alpha-actinin recognizes homologous molecules in chicken skeletal and cardiac muscles, as well as in intestinal epithelial cells, but fails to recognize the brush-border 95-kdalton polypeptide. Conversely, anti-95-kdalton polypeptide does not recognize gizzard alpha-actinin. On high-resolution SDS polyacrylamide gel electrophoresis, alpha-actinin and brush-border 95-kdalton protein exhibit distinct mobilities. The two proteins also differ in their ability to be extracted in nonionic detergent: epithelial cell immunoreactive alpha-actinin is soluble in NP-40, whereas 95-kdalton protein is insoluble. Finally, two-dimensional peptide mapping of iodinated tryptic peptides, as well as one-dimensional fingerprinting of partial tryptic, chymotryptic, papain, and S. aureus V8 protease digests, have revealed <5% homology between gizzard alpha-actinin and brush-border 95-kdalton polypeptide. The data suggest that there is no major structural homology between gizzard alpha-actinin and brush-border 95-kdalton protein. We conclude that it is unlikely that alpha-actinin cross-links actin filaments to the microvillar membrane.