Unusual 5 ' transcript complexity of plectin isoforms: novel tissue-specific exons modulate actin binding activity

Plectin, the most versatile cytolinker identified to date, has essential fu nctions in maintaining the mechanical integrity of skin, skeletal muscle an d heart, as indicated by analyses of plectin-deficient mice and humans, Exp ression of plectin in a vast variety of tissues and cell types, combined wi th a large number of different binding partners identified at the molecular level, calls for complex mechanisms regulating gene transcription and expr ession of the protein. To investigate these mechanisms, we analyzed the tra nscript diversity and genomic organization of the murine plectin gene and f ound a remarkable complexity of its 5'-end structure. An unusually high num ber of 14 alternatively spliced exons, 11 of them directly splicing into pl ectin exon 2, were identified. Analysis of their tissue distribution reveal ed that expression of a few of them is restricted to tissues such as brain, or skeletal muscle and heart. In addition, we found two short exons tissue -specifically spliced into a highly conserved set of exons encoding the N-t erminal actin binding domain (ABD), common to plectin and the superfamily o f spectrin/dystrophin-type actin binding proteins. Using recombinant protei ns we show that a novel ABD version contained in the muscle-specific isofor m of plectin exhibits significantly higher actin binding activity than othe r splice forms, This fine tuning mechanism based on alternative splicing is likely to optimize the proposed biological role of plectin as a cytolinker opposing intense mechanical forces in tissues like striated muscle.