Identification of a homolog of actin-binding protein, ABP-280, localized at epithelial cell-cell boundaries in hydra

ABP-280 cross-links actin filaments and connects membrane glycoproteins and actin filaments in mammalian blood cells. We isolated cDNA clones for ABP-280 from hydra by screening a cDNA expression library with an antiserum against a cell membrane-enriched crude fraction. The amino acid se-quences deduced from the cDNA clones showed significant similarity to human ABP-280, including intramolecular repetition of a unique sequence motif, suggesting similar tertiary structure and molecular activity. Monospecific antibodies were purified from the antiserum, by affinity to fusion proteins produced from the cDNA clones, and used for immunohistochemistry. Specific staining was detected at epithelial cell-cell boundaries in the hydra. The signals were restricted to the subapico-lateral regions corresponding to the locations of septate junctions. These results suggest that the identified molecule is a possible component of the septate junction, and presumably connects membrane cell adhesion molecules to actin filaments in the epithelial cells of hydra. This is the first report describing the subcellular distribution of ABP-280-related molecules in epithelial cells.