Plectin

Plectin is a giant protein (c500 kDa) found in nearly all mammalian cells which acts as a link between the three main components of the cytoskeleton: actin microfilaments, microtubules and intermediate filaments. In addition plectin links the cytoskeleton to junctions found in the plasma membrane that structurally connect different cells. By holding these different networks together plectin plays an important role in maintaining the mechanical integrity and viscoelastic properties of tissues.

Structure
The structure of plectin is thought to be a dimer consisting of a central coiled coil of alpha helices connected to large globular domains at each terminus. These globular domains are responsible for connecting plectin to its various cytoskeletal targets. The carboxy-terminal domain is made of 6 highly homologous repeating regions. The subdomain between regions five and six of this domain is known to connect to the intermediate filaments cytokeratin and vimentin. At the opposite end of the protein, in the N-terminal domain, a region has been defined as responsible for binding to actin. In 2004 the exact crystal structure of this actin binding domain(ABD) was determined in mice and shown to be comprised of two calponin homology(CH) domains.

Function
Through the use of gold-immunoelectron microscopy, immunoblotting and immunofluoresence experiments plectin has been found to associate with all three major components of the cytoskeleton. With the use of microscopy especially plectin has been shown to directly connect microtubules to intermediate filaments as well as to each other. While plectin has been observed to mediate interactions between actin and intermediate filaments and associate with each independently, a direct linkage by plectin between these two filaments has not been completely proven. It may be that plectin actually connects to proteins associated with each rather than directly. Besides serving as a linker protein between the main elements of the cytoskeleton, plectin also forms connections between other cytoskeletally related proteins as well. Plectin has been shown to directly link Myosin II motor proteins and intermediate filaments. In in vitro assays plectin has been found to bind subplasma membrane skeleton proteins such as alpha-spectrin and fodrin. Over the past decade plectin has been identified as an important component linking the cytoskeleton to intercellular junctions which enable the structural integrity of a tissue as a whole. Two such junctions, Desmosomes and Hemidesmosomes which link intermediate filament networks between cells have been shown to associate with plectin. Plectin has been revealed to localize to the desmosomes and in vitro studies have shown that it can form bridges between the desmosome protein, desmoplakin and intermediate filaments. In hemidesmosomes plectin has been shown to interact with the integrin β4 subunits of the hemidesmosome plaque and function in a clamp like manner to crosslink the intermediate filament, cytokeratin to the junction.