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

The polypeptide subunits of the 10 nm intermediate filaments (IFs) in epithelial cells can be grouped into two large multigene families, termed type I and type II cytokeratins (CKs) which are expressed in stoichiometrically equal amounts in a tissuespecific and cell type-restricted pattern (Fuchs and Weber, 1994; Moll, 1998). CK filaments (CKFs) are considered to be particularly important for epithelial resilience to mechanical stress by forming a stable network which is attached to specific cell-cell contacts of the desmosome type (Schmidt et al., 1994; Bornslaeger et al., 1998; Fuchs and Cleveland, 1998). Disturbance of this supracellular network results in reduced tissue coherence and increased cell fragility as it occurs in several blister-forming genodermatoses which are caused by dominant negative-acting CK mutants (Fuchs and Weber, 1994; McLean and Lane, 1995; Fuchs and Cleveland, 1998). The stability of CKFs is reflected by their remarkable biochemical properties, i.e. their resistance to non-ionic detergents, their insolubility in high salt buffers, and the strong interactions of their subunits forming heterodimers (Fuchs and Weber, 1994; Herrmann and Aebi, 1998). Lateral exchange of subunits enabling the continuous maintenance of the entire filament network (Miller et al., 1991, 1993) and the scarce evidence for CK-specific motor proteins further underscore the static nature of CKFs and distinguishes them from the actinbased microfilaments and tubulin-based microtubules. Given that the stabilising properties of CKFs fulfil important functions in epithelial tissue integrity it is at the same time important that they are dynamic structures to avoid interference with processes that are needed for tissue homeostasis and maintenance such as mitosis, stratification, wound healing and migration, or with other more specialised functions such as secretion. Exactly how these two opposing principles are realised is not known although various levels of regulation may be operating that affect filament dynamics by influencing filament organisation, expression of distinct CK pairs with specific dynamic properties, association with regulatory polypeptides and protein modification such as phosphorylation and glycosylation (Eriksson et al., 1992; Skalli et al., 1992; Fuchs and Weber, 1994; McGowan and Coulombe, 1998; Omary et al., 1998). Studies trying to elucidate in vivo dynamics of cellular elements often deduce sequential reaction patterns from results 4521 Journal of Cell Science 112, 4521-4534 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JCS0841