Plectin interacts with the rod domain of type III intermediate filament proteins desmin and vimentin.

Plectin is a versatile cytolinker protein critically involved in the organization of the cytoskeletal filamentous system. The muscle-specific intermediate filament (IF) protein desmin, which progressively replaces vimentin during differentiation of myoblasts, is one of the important binding partners of plectin in mature muscle. Defects of either plectin or desmin cause muscular dystrophies. By cell transfection studies, yeast two-hybrid, overlay and pull-down assays for binding analysis, we have characterized the functionally important sequences for the interaction of plectin with desmin and vimentin. The association of plectin with both desmin and vimentin predominantly depended on its fifth plakin repeat domain and downstream linker region. Conversely, the interaction of desmin and vimentin with plectin required sequences contained within the segments 1A-2A of their central coiled-coil rod domain. This study furthers our knowledge of the interaction between plectin and IF proteins important for maintenance of cytoarchitecture in skeletal muscle. Moreover, binding of plectin to the conserved rod domain of IF proteins could well explain its broad interaction with most types of IFs.

The Z-disc proteins myotilin and FATZ-1 interact with each other and are connected to the sarcolemma via muscle-specific filamins.

Myotilin and the calsarcin family member FATZ-1 (also called calsarcin-2 or myozenin-1) are recently discovered sarcomeric proteins implicated in the assembly and stabilization of the Z-discs in skeletal muscle. The essential role of myotilin in skeletal muscle is attested by the observation that certain forms of myofibrillar myopathy and limb girdle muscular dystrophy are caused by mutations in the human myotilin gene. Here we show by transfection, biochemical and/or yeast two-hybrid assay that: (1) myotilin is able to interact with the C-terminal region of FATZ-1 and that the N- or C-terminal truncations of myotilin abrogate binding; (2) myotilin can also interact with another calsarcin member, FATZ-2 (calsarcin-1, myozenin-2); (3) myotilin and FATZ-1 bind not only to the C-terminal region of filamin-C containing the Ig repeats 19-24, but also to the other two filamins, filamin-A and filamin-B, as well as the newly identified filamin-Bvar-1variant; (4) the binding of myotilin to filamin-C involves binding sites in its N-terminal region, whereas FATZ-1 associates with filamin-C via sequences within either its N- or C-terminal region; and finally, (5) the C-terminal region of filamin-C like filamin-B and filamin-Bvar-1, shows binding activity with the beta1A integrin subunit. Our findings further dissect the molecular interactions within the Z-disc that are essential for its organization, and provide evidence for a novel connection between Z-disc proteins and the sarcolemma via filamins and beta1 integrins. These data shed new light on the complex organization of the Z-disc that is highly relevant to understanding muscular dystrophies.