Structural biology of hemidesmosomesHemidesmosomes are protein complexes that mediate the stable attachement of basal cells to the underlying basement membrane in epithelial tissues, such as the skin (Fig 1A). They provide a link between the extracellular matrix and the intermediate filament cytoskeleton.
Hemidesmosomes contain the following proteins: integrin α6β4, plectin, BPAG1e (also known as BP230), BP180 (known as Collagen XVII), and the tetraspanin CD151 (Fig 1B).
To understand the organization and regulation of hemidesmosomes, we study the structure of the proteins that form these complexes and the structural basis of the interactions that they establish. We also aim at understanding how hemidesmosomal proteins are altered in some types of a blistering diseases named epidermolysis bullosa.
Figure 1. Schematic representation of the components of the hemidesmosomes and the interactions that they stablish with each other.
Integrin α6β4Integrins 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 (~1000 residues) that contains a Calx-β domain and four "fibronectin type III" domains (FnIII-1 to FnIII-4).
We have determined the 3D structure of all these five globular domains. These include the crystal structures of the Calx-β, the first pair of FnIII domain (FnIII-1,2), and the individual structures of the third and fourth FnIII domains.
To elucidated the structure of the FnIII-3,4 region we have used hybrid methods. In that multi-resolution approach we combined the crystal structures of the FnIII-3 and FnIII-4 with SAXS, and double electron-electron resonance (DEER-EPR) data.
Our contributions to the study of the structure of the integrin β4 subunit are summarized in Figure 2.
Figure 2. Structure of the cytoplasmic tails of integrin α6β4.
PlectinPlectin belongs to the plakin protein family of cytoskeletal linkers. In hemidesmosomes plectin provides a direct link between the integrin α6β4 and the cytokeratin filaments.
Plectin is a large protein (~500 kD) with a tripartite organization: it contains N- and C-terminal regions that harbor protein-protein interaction sites and a central rod domain involved in oligomerization (Fig 3).
We have extensively characterized the N-terminal region of plectin. This segment consists of an actin binding domain (ABD) and a region termed the plakin domain that is conserved in most plakins.
We have solved the crystal structure of the isolated ABD. We have also solve the 3D structure of the ABD bound to the FnIII-1,2 of integrin α6β4 (Fig 1C). Our structure of the plectin-α6β4 comlex revealed key determinants for the interaction. It also showed that two single amino-acid substitutions in the integrin that cause epidermolysis bullosa disrupt the binding interface.
The plakin domain is built up of nine spectrin repeats (SR1 to SR9) and a SH3 domain embedded in the SR5. Using X-ray crystallography we have solved the structures of the regions SR1-SR2, SR3-SR4, SR4-SR5-SH3, SR5-SR6, SR7-SR8, and SR7-SR9. Collectively our structures cover the repeats of the plakin domain and reveal significant structural variation along these region.
In addition, we have used SAXS to elucidate the 3D structure of the SR3-SR9. These region forms an extended and slightly bent rod-like structure that spans ~340 Å.
Our contributions to the study of the structure of plectin are summarized in Figure 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 includes the ABD and the complete plakin domain (SR1 to SR9).