58 Histochem Cell Biol (2008) 130:55–70 Fig. 3 Transmembrane proteins of the tight junction. a Murine occlu- din is a 521 amino acid protein. The Wrst extracellular domain may be involved in cell–cell interaction containing a high tyrosine and glyine domain (triangles), acidic amino acids (orange-red), basic amino acids (green), neutral amino acids (cream). A second functional domain was assigned to the carboxy terminal 150 amino acids which appears to be responsible for the association of occludin with ZO-1. The occludin amino acid sequence is highly conserved between species (Ando-Akat- suka et al. 1996). b Claudins are four transmembrane domain proteins, containing two extracellular and one intracellular loop and a N- and C-terminal cytoplasmic domain. Claudins are a multigene family with so far 24 members identiWed in diverse species. In the Wrst extracellular loop, claudins have a conserved common motif, GLWxxC(8–10aa)C (blue). Claudins further contain a PDZ-binding motif at the C terminus (orange) capable of bining to TJ plaque proteins like ZO-1. The clau- dins show a isotype-speciWc tissue expression pattern. c JAM-1 is an integral membrane protein expressed in endothelial and epithelial for the JAM proteins, a role in the assembly of TJs and regulation of paracellular permeability could be deciphered by ectopic transfection experiments (Cohen et al. 2001). Interaction of JAM-1 with occludin, so far not determined whether direct or indirect in nature, has been shown (Liu et al. 2000). TJ plaque proteins and the coordination of signaling at the TJ In addition to their prime function as a regulated permeability barrier in the paracellular pathway and as fence in the plane of the membrane, TJs play a pivotal role in organizing such diverse processes as morphogenesis, cell polarity, cell proliferation, and diVerentiation, which require the coordi- nation of signals impinging on and emanating from the 123 cells. Its extracellular domain can dimerize and bind homophilically. The intracellular domain (and in particular a PDZ-binding motif) en- able JAM-1 to interact with structural and signaling proteins. JAM-1 is localized at the tight junctions of epithelial and endothelial cells and is involved in the regulation of junctional integrity and permeability. The polypeptide sequence of 299 amino acids has the typical feature ob- served in a type I integral membrane protein. A putative signal peptide may be cleaved between Leu23 and Val24 (pink), leaving 215 residues in the extracellular domain of the mature protein. A stretch of 17 hydrophobic residues (Ile 239 ! Phe 255) presents a potential trans- membrane region, and there are 45 residues in the cytoplasmic domain. The extracellular portion contains two domains with intrachain disul- Wde bonds typical of immunoglobulin-like loops of the V-type. Numer- ous PKC (blue) and Casein kinase II (yellow) phophorylation sites have been detected involved in platelet activation processes (Sobocka et al. 2000). Cys-43, -111, 146, -214 form dimer containing disulWde bridges and are labeled in black plasma membrane. Mounting evidence suggests that the cytosolic TJ plaque is one of the sites in which such signal- ing is coordinated (Ebnet 2008; Gonzalez-Mariscal et al. 2003; Schneeberger and Lynch 2004). Since identiWcation in 1986 of zonula occludens (ZO)-1 as the Wrst TJ-associ- ated protein (Stevenson et al. 1986), almost 30 additional proteins have been described associated with the cytoplas- mic aspect of TJs (Schneeberger and Lynch 2004). They can be grouped into two major categories: The Wrst are the peripherally associated scaVolding proteins like ZO-1 (ZO- 2, ZO-3, AF6, and cingulin) that appear to organize the transmembrane proteins and couple them to other cytoplas- mic proteins and to actin microWlaments. The second are numerous “signaling” proteins (ZONAB, RhoA, RalA, and Raf-1) proposed to be involved in junction assembly, bar- rier regulation, gene transcription, and perhaps other, pres- ently undeWned pathways.