Trophinin and tastin form a cell adhesion molecule complex that potentially mediates an initial attachment of the blastocyst to uterine epithelial cells at the time of implantation. Trophinin and tastin, however, do not directly bind to each other, suggesting the presence of an intermediary protein. The present study identifies a cytoplasmic protein, named bystin, that directly binds trophinin and tastin. Bystin consists of 306 amino acid residues and is predicted to contain tyrosine, serine, and threonine residues in contexts conforming to motifs for phosphorylation by protein kinases. Database searches revealed a 53% identity of the predicted peptide sequence with the Drosophila bys (mrr) gene. Direct proteinprotein interactions of trophinin, tastin, and bystin analyzed by yeast two-hybrid assays and by in vitro protein binding assays indicated that binding between bystin and trophinin and between bystin and tastin is enhanced when cytokeratin 8 and 18 are present as the third molecule. Immunocytochemistry of bystin showed that bystin colocalizes with trophinin, tastin, and cytokeratins in a human trophoblastic teratocarcinoma cell, HT-H. It is therefore possible that these molecules form a complex and thus are involved in the process of embryo implantation.Embryo implantation is a process that depends on the coordinated development of the embryo and differentiation of the uterus to the receptive state (1-4). A two-way interaction between the blastocyst and uterus is essential for successful implantation and subsequent decidualization (5). In the mouse, the first conspicuous sign of the implantation is an increased endometrial vascular permeability at the site of blastocyst apposition, and this coincides with the initial attachment reaction (3, 6). This attachment is followed by adherence and penetration by trophoblasts cells through the underlying basement membrane and results in proliferation and differentiation of stromal cells into decidual cells. Numerous factors including growth factors (7), cytokines (8), homeotic genes (9), and prostaglandin (10, 11) have been implicated in implantation process. Among these, null mutations of leukemia inhibitory factor and Hoxa-10 genes result in defective implantation (8, 9), and a prostaglandin regulating cyclooxigenase 2 gene knock-out results in multiple failures of female reproductive processes including implantation (10, 11).To understand the molecular mechanisms underlying embryo implantation, identification and characterization of specific molecules responsible for the initial attachment of the embryo and subsequent invasion of the trophoblasts to the uterus are essential. However, such analysis has been difficult because of the absence of appropriate in vitro models for implantation. In this regard two human cell lines, a trophoblastic teratocarcinoma, HT-H (12), and an endometrial adenocarcinoma, SNG-M (13), are noteworthy, because the interaction between these two cell types appears to mimic that of trophoblasts and endometrial epithelial cells parti...