Cytoskeletal Linker Proteins - Plakins

Plakins are large multidomain proteins that crosslink different types of cytoskeletal filaments and connect them to cell junctions on the plasma membrane. They organize cytoskeletal components by linking microfilaments, microtubules, and intermediate filaments to each other and to cell-matrix and cell adhesion complexes, maintaining tissue integrity and cellular organization.

Plakins contain distinct binding domains for each cytoskeletal filament type. The characteristic plakin domain directs attachment to target regions, while a coiled-coil rod domain enables dimerization. Plakin repeats, linker subdomains, and glycine-serine-arginine domains combine in different arrangements to create proteins that bind microfilaments, microtubules, intermediate filaments, and membrane proteins like integrins.

Desmoplakins are plakin proteins found in desmosomes, which are cell junctions in epithelial tissues. They contain plakin repeats followed by a glycine-serine-arginine domain. These proteins help maintain epithelial tissue integrity by anchoring and organizing cytoskeletal filaments at cell-cell adhesion sites.

Recent research reveals plakins function in non-epithelial cells including neurons, cardiac muscle, and skeletal muscle. Alternative splicing of the plakin gene produces tissue-specific isoforms with unique domain organizations. These isoforms interact with other cytoskeletal filaments and serve as scaffolds for signaling complexes, modulate vesicle trafficking, and interact with signal transducers.

Plectin is a versatile cytoskeletal linker comprising distinct and overlapping domains that bind all three types of cytoskeletal filaments and membrane proteins like integrins. Unlike plakins restricted to epithelial tissues, plectin functions across diverse cell types. Mutations in plectin genes cause epidermolysis bullosa simplex with muscular dystrophy, resulting in fragile skin and progressive muscle weakness.

Defects in plakin genes cause disorders affecting skin, neuronal tissue, and cardiac and skeletal muscle. Humans have seven plakin proteins including desmoplakin, envoplakin, epiplakin, bullous pemphigoid antigen 1, microtubule-actin crosslinking factor 1, periplakin, and plectin. Mutations in these genes result in skin disorders and muscular system diseases, with plectin mutations specifically causing epidermolysis bullosa simplex with muscular dystrophy.

Beyond crosslinking cytoskeletal filaments, plakins interact with signal transducers and serve as scaffolds for signaling complexes. They modulate vesicle trafficking and connect cytoskeletal components to cell-matrix and cell adhesion complexes. These functions enable plakins to coordinate cellular responses and facilitate intracellular transport processes essential for cell function and tissue maintenance.