The Structure of the Plakin Domain of Plectin Reveals an Extended Rod-like Shape

Abstract

Plakins are large multi-domain proteins that interconnect cytoskeletal structures. Plectin is a prototypical plakin that tethers intermediate filaments to membrane-associated complexes. Most plakins contain a plakin domain formed by up to nine spectrin repeats (SR1-SR9) and an SH3 domain. The plakin domains of plectin and other plakins harbor binding sites for junctional proteins. We have combined x-ray crystallography with small angle x-ray scattering (SAXS) to elucidate the structure of the plakin domain of plectin, extending our previous analysis of the SR1 to SR5 region. Two crystal structures of the SR5-SR6 region allowed us to characterize its uniquely wide inter-repeat conformational variability. We also report the crystal structures of the SR7-SR8 region, refined to 1.8 Å, and the SR7-SR9 at lower resolution. The SR7-SR9 region, which is conserved in all other plakin domains, forms a rigid segment stabilized by uniquely extensive inter-repeat contacts mediated by unusually long helices in SR8 and SR9. Using SAXS we show that in solution the SR3-SR6 and SR7-SR9 regions are rod-like segments and that SR3-SR9 of plectin has an extended shape with a small central kink. Other plakins, such as bullous pemphigoid antigen 1 and microtubule and actin cross-linking factor 1, are likely to have similar extended plakin domains. In contrast, desmoplakin has a two-segment structure with a central flexible hinge. The continuous versus segmented structures of the plakin domains of plectin and desmoplakin give insight into how different plakins might respond to tension and transmit mechanical signals.Plakins are a family of very large proteins that interconnect and organize the intermediate filaments (IF), 4 microtubules, and microfilaments of the cytoskeleton and tether them to membrane-associated structures (1, 2). So far, seven plakins have been described in mammals; these are plectin, bullous pemphigoid antigen 1 (BPAG1), desmoplakin, microtubule and actin cross-linking factor 1 (MACF1, also known as ACF7), envoplakin, periplakin, and epiplakin. Invertebrates have a more reduced plakin repertoire; for example Caenorhabditis elegans and Drosophila melanogaster each have a single plakin gene encoding VAB-10 and Shot (also known as Short Stop or kakapo), respectively. Most of the plakin genes produce multiple isoforms that increase the structural and functional versatility of these proteins.Plectin is expressed in a large variety of cell types in which it acts as a highly polyvalent cytolinker that contributes to cell adhesion and the organization of the cytoskeleton. Plectin cross-links IFs to microtubules and actin filaments and mediates the attachment of IFs to cell-cell and cell-matrix junctional complexes such as hemidesmosomes, desmosomes, Z-lines, and focal contacts. Plectin also connects IFs to organelles such as the nucleus and mitochondria (3). Defects in the PLEC gene cause various forms of the blistering disease epidermolysis bullosa simplex (EBS), which may occur only with skin fragility, as found in EBS Og...

Desmosomes and Intermediate Filaments: Their Consequences for Tissue Mechanics

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