Epiplakin, a Novel Member of the Plakin Family Originally Identified as a 450-kDa Human Epidermal Autoantigen

Abstract

A 450-kDa human epidermal autoantigen was originally identified as a protein that reacted with the serum from an individual with a subepidermal blistering disease. Molecular cloning of this protein has now shown that it contains 5065 amino acids and has a molecular mass of 552 kDa. As reported previously this protein, which we call epiplakin, belongs to the plakin family, but it has some very unusual features. Epiplakin has 13 domains that are homologous to the B domain in the COOH-terminal region of desmoplakin. The last five of these B domains, together with their associated linker regions, are particularly strongly conserved. However, epiplakin lacks a coiled-coil rod domain and an amino-terminal domain, both of which are found in all other known members of the plakin family. Furthermore, no dimerization motif was found in the sequence. Thus, it is likely that epiplakin exists in vivo as a single-chain structure. Epitope mapping experiments showed that the original patient's serum recognized a sequence unique to epiplakin, which was not found in plectin. Immunofluorescence staining revealed the presence of epiplakin in whole sheets of epidermis and esophagus, in glandular cells of eccrine sweat and parotid glands and in mucous epithelial cells in the stomach and colon. A 450-kDa human epidermal autoantigen was originally identified as a protein that reacted with the serum from an individual with a subepidermal blistering disease. Molecular cloning of this protein has now shown that it contains 5065 amino acids and has a molecular mass of 552 kDa. As reported previously this protein, which we call epiplakin, belongs to the plakin family, but it has some very unusual features. Epiplakin has 13 domains that are homologous to the B domain in the COOH-terminal region of desmoplakin. The last five of these B domains, together with their associated linker regions, are particularly strongly conserved. However, epiplakin lacks a coiled-coil rod domain and an amino-terminal domain, both of which are found in all other known members of the plakin family. Furthermore, no dimerization motif was found in the sequence. Thus, it is likely that epiplakin exists in vivo as a single-chain structure. Epitope mapping experiments showed that the original patient's serum recognized a sequence unique to epiplakin, which was not found in plectin. Immunofluorescence staining revealed the presence of epiplakin in whole sheets of epidermis and esophagus, in glandular cells of eccrine sweat and parotid glands and in mucous epithelial cells in the stomach and colon. Clarification of the basic structure of desmoplakin has been followed by the identification of many related proteins, such as BPAG1,1 plectin, envoplakin, and periplakin (1Green K.J. Parry D.A.D. Steinert P.M. Virata M.L.A. Wagner R.M. Angst B.D. Nilles L.A. J. Biol. Chem... 1990; 265: 2603-2612Google Scholar, 2Tanaka T. Parry D.A.D. Klaus-Kovtun V. Steinert P.M. Stanley J. J. Biol. Chem... 1991; 266: 12555-12559Google Scholar, 3Sawamura D. Li K. Chu M.L. Uitto J. J. Biol. Chem... 1991; 266: 17784-17790Google Scholar, 4Wiche G. Becker B. Luber K. Weitzer G. Castanon M.J. Hauptmann R. Stratowa C. Stewart M. J. Cell Biol... 1991; 114: 83-99Google Scholar, 5McLean W.H.I. Pulkkinen L. Smith F.J.D. Rugg E.L. Lane E.B. Bullrich F. Burgeson R.E. Amano S. Hudson D.L. Owaribe K. McGrath J.A. McMillan J.R. Eady R.A.J. Leigh I.M. Christiano A.M. Uitto J. Genes Dev... 1996; 10: 1724-1735Google Scholar, 6Ruhrberg C. Hajibagheri M.A.N. Simon M. Dooly T.P. Watt F.M. J. Cell Biol... 1996; 134: 715-729Google Scholar, 7Ruhrberg C. Hajibagheri M.A.N. Parry D.A.D. Watt F.M. J. Cell Biol... 1997; 139: 1835-1849Google Scholar). These proteins form a family known as the “plakin family” (8Uitto J. Pulkkinen L. Smith F.J.D. McLean W.H.I. Exp. Dermatol... 1996; 5: 237-246Google Scholar). Almost all members of this family have a common structure, with predicted globular amino-terminal and COOH-terminal domains that ar