Figure 1. Integrins. Integrin heterodimers are transmembrane receptors that are composed of and subunits. The cytoplasmic domains of all and subunits except 4 are short (around 30 amino acids). Some but not all subunits are posttranslationally cleaved into two fragments that are connected by disulfide bonds (indicated by a bracket and marked with an ‘S’: the position of the cleavage site in most integrin subunits is relatively close to the membrane as indicated, however, it is further away from the membrane in 4 and E). The table on the right summarizes some integrin ligands. Integrins interact with secreted molecules and with membrane-bound counter-receptors. Most of the counter-receptors are members of the Ig-superfamily. ADAMs (A Disintegrin And Metalloproteinase) are a family of cell surface molecules with homology to both disintegrins and metalloproteinases. Not shown are pathogens and viruses that bind to integrins (Kerr, 1999). Bsp-1: bone sialoprotein-1; C3bi: inactivated form of C3b component of complement; Co: collagen; factor X: coagulation factor X; Fb: fibrinogen; Fn: fibronectin; Ln: laminin; MMP2: matrix metalloproteinase 2; NIF: neutrophil inhibitory factor; Opn: osteopontin; proThr: prothrombin; Tn-C: tenascin-C; Tsp-1: thrombospondin-1; Vn: vitronectin; vWF: von Willebrand factor; dis: disintegrins (integrin antagonists from snake venoms).

Figure 2. Ig-CAMs. A subset of all known Ig-CAMs is shown to highlight characteristic features. Ig-CAMs contain varying numbers of Ig-domains in their extracellular domain, either alone or in combination with other structural motifs such as fibronectin type III (FNIII) repeats that have originally been identified in the ECM component fibronectin. Ig-CAMs are linked to the membrane either by a transmembrane domain or via a glycosylphosphatidylinositol (GPI) anchor. Some, such as NCAM, exist in both forms.

Figure 3. Cadherins. The cadherin superfamily is subdivided into classical cadherins, protocadherins, seven pass G-type cadherins, desmosomal cadherins and a few others that are more difficult to classify. All contain in their extracellular domain repetitions of calcium-binding motifs, the EC domains. Interestingly, some cadherin genes form gene clusters with many classical cadherin genes on chromosome 16, and the desomosomal cadherin genes on chromosome 18. The most striking arrangement is found in the protocadherin gene family. Approximately 52 human cadherins are organized into three tandem gene clusters (Pcdh,,) on chromosome 5q31. The arrangement of the clusters resembles the genomic organization of the immunoglobulin and T-cell receptor clusters.

Figure 4. Hemidesmosomes. Hemidesmosomes are specialized junctional complexes mediating adhesion of epithelial cells to the basement membranes. Some of the known molecular components of a hemidesmosome are shown. According to the molecular composition and appearance in the electron microscope, different hemidesmosomal substructures have been defined such as the inner and outer plaque (indicated on the right).

Figure 5. Dystrophin glycoprotein complex. The dystrophin glycoprotein complex is a multisubunit transmembrane complex that links the muscle fiber basement membrane to the sarcolemmal plasma membrane. The -dystroglycan component of the complex mediates interactions with basement membrane components such as laminin, perlecan and agrin. Dystrophin mediates interactions to the cytoskeleton.