Abstract. Hemidesmosomes (HDs) mediate cell adhesion to the extracellular matrix and have morphological association with intermediate-sized filaments (IFs) through cytoplasmic plaques . Though several proteins have been located in HDs, most of them have not been well characterized, with the exception of the 230-kD antigen of bullous pemphigoid (BP), an autoimmune skin blistering disease. Only recently we have succeeded in isolating HDs from bovine corneal epithelial cells and in identifying five major components on SDS-PAGE (Owaribe K., Y. Nishizawa, and W. W. Franke . 1991 . Exp. Cell Res . 192 :622-630) . In this study we report on immunological characterization of one of the major components, termed HDl, with an H EMIDESMOSOMES (HDs)' and focal adhesions are cell junctions that mediate adhesion to the extracellular matrix (ECM) . These two plasma membrane junction types have common ultrastructural features ; e.g., both are associated with cytoskeeeool elements through electron-dense cytoplasmic plaques. They thus provide us with models to explore the mechanisms of transmembrane connection between the ECM and the cytoskeleton. In focal adhesions associated with actin-containing microfilaments (for reviews see references 2 and 50), a number ofproteins are known to be involved, including ECM receptors of the integrin family (5,12,17,33) and plaque components such as talin (1) and vinculin (29) . Yet our knowledge about the molecular architecture of the ECM-to-microfilaments junctions is still incomplete.The HD, primarily found in the basement membrane zone (BMZ) of stratified and complex epithelia, has a complex structure: a cytoplasmic plaque associated with intermediate filaments (IFs), a subbasal dense plate with anchoring filaments in the basal lamina, and anchoring fibrils in collagenous connective tissues (6,16,39). The morphological appearance is essentially that of half of the desmosome of intercellular junctions. However, although several des- apparent molecular mass of 500 kD. Immunofluorescence microscopy showed colocalization of HDl with BP antigen at the basement membrane zone of those tissues that have typical HDs, including skin epidermis, corneal and tracheal epithelia, and myoepithelium . In cultured keratinocytes, HDl demonstrated colocalization with BP antigen in the precise way, while being absent from focal adhesions . Immunoelectron microscopy revealed that an epitope of HDl was located on the cytoplasmic side of HDs . Taking all these results together, we conclude that HDl is a new hemidesmosomal component . Interestingly, HDl also exists in endothelial and glial cells, which lack typical HDs . mosomal proteins have been identified (e.g., references 4, 26; for review see reference 35) since the establishment of appropriate isolation procedures (e.g., reference 36), none of them have been found in HDs (13, 35) . Thus, in marked contrast to other celljunctions, little information is available about the molecular components of HDs, with the exception ofthe well-characterized 230-kD antigen r...