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© The Rockefeller University Press, 0021-9525/1998//1539 $5.00
The Journal of Cell Biology, Volume 141, Number 7, , 1998 1539-1550

Articles

Claudin-1 and -2: Novel Integral Membrane Proteins Localizing at Tight Junctions with No Sequence Similarity to Occludin



Mikio Furuse*, Kohji Fujita*,{ddagger}, Takashi Hiiragi*, Kazushi Fujimoto§, and Shoichiro Tsukita*

* Department of Cell Biology, {ddagger} Department of Neurosurgery, and § Department of Anatomy, Faculty of Medicine, Kyoto University, Sakyo-ku, Kyoto 606, Japan

Occludin is the only known integral membrane protein localizing at tight junctions (TJ), but recent targeted disruption analysis of the occludin gene indicated the existence of as yet unidentified integral membrane proteins in TJ. We therefore re-examined the isolated junction fraction from chicken liver, from which occludin was first identified. Among numerous components of this fraction, only a broad silver-stained band ~22 kD was detected with the occludin band through 4 M guanidine-HCl extraction as well as sonication followed by stepwise sucrose density gradient centrifugation. Two distinct peptide sequences were obtained from the lower and upper halves of the broad band, and similarity searches of databases allowed us to isolate two full-length cDNAs encoding related mouse 22-kD proteins consisting of 211 and 230 amino acids, respectively. Hydrophilicity analysis suggested that both bore four transmembrane domains, although they did not show any sequence similarity to occludin. Immunofluorescence and immunoelectron microscopy revealed that both proteins tagged with FLAG or GFP were targeted to and incorporated into the TJ strand itself. We designated them as "claudin-1" and "claudin-2", respectively. Although the precise structure/function relationship of the claudins to TJ still remains elusive, these findings indicated that multiple integral membrane proteins with four putative transmembrane domains, occludin and claudins, constitute TJ strands.

Abbreviations used in this paper: GFP, green fluorescence protein; ORF, open reading frame; pAb, polyclonal antibody; TJ, tight junctions.

Address all correspondence to Shoichiro Tsukita, M.D. and Ph.D., Department of Cell Biology, Faculty of Medicine, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan. Tel.: 81-75-753-4372. Fax: 81-75-753-4660. E-mail: htsukita{at}mfour.med.kyoto-u.ac.jp


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