Structural biology of hemidesmosomes

Hemidesmosomes are protein complexes that mediate the stable attachement of basal cells to the underlying basement membrane in epithelial tissues, such as the skin. They provide a link between the extracellular matrix and the intermediate filament cytoskeleton. Hemidesmosomes contain at least 5 components: the α6β4 integrin, plectin, the bullous pemphigoid antigens BPAG1e (also known as BP230) and BP180, and the tetraspanin CD151 (Fig 1).

Figure 1. Schematic representation of the components of the hemidesmosomes and the interactions that they stablish with each other.

Integrin α6β4

Integrins are a family of hetero-dimeric transmembrane receptors that mediate cell adhesion and bi-directional signal transduction. The α6β4 integrin is a laminin receptor. The β4 subunit has an unusual cytoplasmic region, which is ~1000 residues long and contains four fibronectin type III (FnIII) domains and a Calx-β domain. We have determined the crystal structure of the first tandem pair of FnIII domains (FnIII-1,2) of β4, a region that it is essential for the binding of β4 to plectin. In addition we have solved the crystal structure of the Calx-β domain, which reveals that it does not bind calcium in contrast to other Calx-β domains. Some of our contributions to the structural characterization of the integrin β4 subunit are summarized in Fig 2.

Figure 2. Structure of the cytoplasmic tails of integrin α6β4.

Plectin

Plectin is a versatile cytoskeletal linker that belongs to the plakin protein family. In hemidesmosomes plectin provides a direct link between the integrin α6β4 and the cytokeratin system. Plectin is a large molecule (~500 kD) with a tripartite organization: it contains N- and C-terminal regions that harbor multiple protein-protein interaction sites and a central rod domain involved in oligomerization (Fig 3).

We have focused our studies on the N-terminal region of plectin, which consists of an actin binding domain (ABD) and a region termed the plakin domain that is conserved in most of the members of the plakin family. We have solved the crystal structures of the ABD. The ABD is built up of two calponin homology domains (CH1 and CH2). The plakin domain is build up of nine spectrin repeats (SR1 to SR9) and a SH3 domain inserted in the SR5. We have solved the crystal structures of the regions SR1-SR2, SR3-SR4, SR4-SR5-SH3. In addition, we have used SAXS to validate composite models of the SR3-SR5 region. Our contributions to the study of the structure of plectin are summarized in Fig 3.

Figure 3. Structure of Plectin: overall organization of the three major regions of plectin (top). Schematic representation of the sub-domains that compose the N-terminal region (middle). Ribbon diagrams of the 3D structures of the N-terminal region of plectin solved by our group, which cover from the ABD to the SR5.