Plectin, the most versatile cytolinker identified to date, has essential functions in maintaining the mechanical integrity of skin, skeletal muscle and heart, as indicated by analyses of plectin-deficient mice and humans. Expression of plectin in a vast variety of tissues and cell types, combined with a large number of different binding partners identified at the molecular level, calls for complex mechanisms regulating gene transcription and expression of the protein. To investigate these mechanisms, we analyzed the transcript diversity and genomic organization of the murine plectin gene and found a remarkable complexity of its 5'-end structure. An unusually high number of 14 alternatively spliced exons, 11 of them directly splicing into plectin exon 2, were identified. Analysis of their tissue distribution revealed 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-terminal actin binding domain (ABD), common to plectin and the superfamily of spectrin/dystrophin-type actin binding proteins. Using recombinant proteins we show that a novel ABD version contained in the muscle-specific isoform of plectin exhibits significantly higher actin binding activity than other 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.

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Abstract. We have determined the complete cDNA sequence of rat plectin from a number of wellcharacterized overlapping lambda gtll clones . The 4,140-residue predicted amino acid sequence (466,481 D) is consistent with a three-domain structural model in which a long central rod domain, having mainly an alpha-helical coiled coil conformation, is flanked by globular NH2-and COOH-terminal domains. The plectin sequence has a number of repeating motifs . The rod domain has five subregions N200-residues long in which there is a strong repeat in the charged amino acids at 10.4 residues that may be involved in association between plectin molecules . The globular COOH-terminal domain has a prominent sixfold tandem repeat, with each repeat having a strongly conserved central region based on nine tandem repeats of a 19-residue motif. The plectin sequence has several marked similarities to that of desmoplakin (Green, K. J., D. A. D. Parry, P M. Steinert, M . L . A. Virata, R. M . Wagner, B. D. Angst, and P LECTIN is an abundant high molecular weight cytomatrix protein found in a wide variety of different cell types. Based mainly on biochemical and immunolocalization studies it has been proposed that plectin plays a role in the cross-linking of intermediate filaments, the interlinking of intermediate filaments with microtubules and microfilaments, and the anchoring of intermediate filaments to the plasma membrane and the nuclear membrane (for reviews see Wiche, 1989;Foisner and Wiche, 1991.). Furthermore, plectin's ability to form networks through self-association may convey structural stability to cytoplasmic areas which are devoid of cytoskeletal filaments. Plectin molecules exist in solution predominantly as dumbbell-shaped ti200-nm-long structures that have a high tendency to aggregate via their globular end domains yielding networklike protein as-1. Abbreviation used in this paper: BP, bullous pemphigoid .

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