Isolation of cDNA Encoding 7H6-Reactive Polypeptide Defines a New Class of Protein with alpha-Helical Coiled-Coil Structure and DA-Box Similar to Yeast Chromosomal Segregation Proteins
7H6 monoclonal antibody was recently developed in our laboratory by im-munizing mice with a bile canaliculus-rich fraction of the rat liver. The anti-body reacted with a novel 155 Kd polypeptide designated 7H6 antigen that specifically localizes at tight junctions of various epithelia. Correlations of the paracellular barrier function of the tight junction with expression of the 7H6 antigen at the cell border have suggested important roles of this polypeptide for the maintenance of tight junctional functions. As the first step for the analysis of the antigen at the molecular level, we isolated a series of cDNA clones encod-ing 7H6-reactive polypeptides. Five clones were isolated by immunoscreening. Among them a clone designated RL5.3 which carries the largest 5.3Kb insert was characterized in this study. Both plaque screening and immunoblotting of the fusion protein produced by the RL5.3 clone with lysogen confirmed that the pro-tein specifically reacts with the 7H6 monoclonal antibody. Using DNA fragmentsof the RL5.3 clone, 21 clones were further identified. Studies with restriction enzymes and probe hybridization revealed that all the cDNA clones were derived from a single class of transcripts. A partial sequence identified one open reading frame with an α-helical coiled-coil structure and highly conserved aspartate (D)- alanine (A) residues with a helix-loop-helix structure corresponding to DA- box. Since this domain has been specifically found in yeast chromosomal segregation proteins (SMC1, CUT3 and CUT14), the polypeptide encoded by the RL5.3 clone provides the first rodent counterpart of these protein family. Yeast is known to be lethal when SMC and CUT proteins are deleted, suggesting essential roles of these proteins for cell cycle progression as a regulator for chromosomal segregation. Identification of a mammalian counterpart of this pro-tein family may give us some clues for a better understanding of fundamental regulatory mechanisms in the function of tigh junctions
