To monitor the desmosome-anchored cytokeratin network in living cells fusion protein HK13-EGFP consisting of human cytokeratin 13 and the enhanced green fluorescent protein was stably expressed in vulvar carcinoma-derived A-431 cells. It is shown for A-431 subclone AK13-1 that HK13-EGFP emits strong fluorescence in fixed and living cells, being part of an extended cytoplasmic intermediate filament network that is indistinguishable from that of parent A-431 cells. Biochemical, immunological and ultrastructural analyses demonstrate that HK13-EGFP behaves identically to the endogenous cytokeratin 13 and is therefore a reliable in vivo tag for this polypeptide and the structures formed by it. Time-lapse fluorescence microscopy reveals that the cytokeratin 13-containing network is in constant motion, resulting in continuous restructuring occurring in single and migratory cells, as well as in desmosome-anchored cells. Two major types of movement are distinguished: (i) oscillations of mostly long filaments, and (ii) an inward-directed flow of fluorescence originating as diffuse material at the cell periphery and moving in the form of dots and thin filaments toward the deeper cytoplasm where it coalesces with other filaments and filament bundles. Both movements are energy dependent and can be inhibited by nocodazole, but not by cytochalasin D. Finally, disassembly and reformation of cytokeratin filament networks are documented in dividing cells revealing distinct and rapidly occurring stages of cytokeratin organisation and distribution.

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