Cytokeratins are expressed in different types of epithelial cells in certain combinations of polypeptides of the acidic (type I) and basic (type II) subfamilies, showing "expression pairs." We have examined in vitro the ability of purified and denatured cytokeratin polypeptides of human, bovine, and rat origin to form the characteristic heterotypic subunit complexes, as determined by various electrophoretic techniques and chemical cross-linking, and, subsequently, intermediate-sized filaments (IFs), as shown by electron microscopy. We have found that all of the diverse type I cytokeratin polypeptides examined can form complexes and IFs when allowed to react with equimolar amounts of any of the type II polypeptides. Examples of successful subunit complex and IF formation in vitro include combinations of polypeptides that have never been found to occur in the same cell type in vivo, such as between epidermal cytokeratins and those from simple epithelia, and also heterologous combinations between cytokeratins from different species. The reconstituted complexes and IFs show stability properties, as determined by gradual "melting" and reassociation, that are similar to those of comparable native combinations or characteristic for the specific new pair combination. The results show that cytokeratin complex and IF formation in vitro requires the pairing of one representative of each the type I and type II subfamilies into the heterotypic tetramer but that there is no structural incompatibility between any of the members of the two subfamilies. These findings suggest that the co-expression of specific pair combinations observed in vivo has other reasons than general structural requirements for IF formation and probably rather reflects the selection of certain regulatory programs of expression during cell differentiation. Moreover, the fact that certain cytokeratin polypeptide pairs that readily form complexes in vitro and coexist in the same cells in vivo nevertheless show preferential, if not exclusive, partner relationships in the living cell points to the importance of differences of stabilities among cytokeratin complexes and/or the existence of extracytokeratinous factors involved in the specific formation of certain cytokeratin pairs.

Cytokeratins are a large multigene family comprising two polypeptide types, i.e. acidic (type I) and basic (type II) ones, which are distinguished on the basis of immunological, peptide mapping, mRNA hybridization, and primary amino acid sequence data. The acidic (type I) cytokeratins can be subdivided into at least two different subtypes on the basis of their carboxy-terminal sequences. Considerable interspecies conservation of sequences exists, even extending to the 3'-non-coding mRNA regions. Different pairs of type I and II cytokeratins show different resistance to dissociation in urea. Sequence differences of the type I cytokeratins containing functional domains may be an explanation of the observed preference of co-expression with certain type II cytokeratins. The distribution of the different type I and II cytokeratins in normal epithelia and in carcinomas is differentiation related and can be used for cell typing and identification. The cell type-specific expression of cytokeratin polypeptides is recognized at both the protein and the mRNA level. The building block of cytokeratin IFs is a heterotypic tetramer, consisting of two type I and two type II polypeptides arranged in pairs of laterally aligned coiled coils. This principle of tetrameric organization is thought to be generally applicable to IFs.

In their progression from the basal to upper differentiated layers of the epidermis, keratinocytes undergo significant structural changes, including establishment of close intercellular contacts. An important but so far unexplored question is how these early structural events are related to the biochemical pathways that trigger differentiation. We show here that β-catenin, γ-catenin/plakoglobin, and p120-Cas are all significantly tyrosine phosphorylated in primary mouse keratinocytes induced to differentiate by calcium, with a time course similar to that of cell junction formation. Together with these changes, there is an increased association of α-catenin and p120-Cas with E-cadherin, which is prevented by tyrosine kinase inhibition. Treatment of E-cadherin complexes with tyrosine-specific phosphatase reveals that the strength of α-catenin association is directly dependent on tyrosine phosphorylation. In parallel with the biochemical effects, tyrosine kinase inhibition suppresses formation of cell adhesive structures, and causes a significant reduction in adhesive strength of differentiating keratinocytes. The Fyn tyrosine kinase colocalizes with E-cadherin at the cell membrane in calcium-treated keratinocytes. Consistent with an involvement of this kinase, fyn-deficient keratinocytes have strongly decreased tyrosine phosphorylation levels of β- and γ-catenins and p120-Cas, and structural and functional abnormalities in cell adhesion similar to those caused by tyrosine kinase inhibitors. Whereas skin of fyn−/− mice appears normal, skin of mice with a disruption in both the fyn and src genes shows intrinsically reduced tyrosine phosphorylation of β-catenin, strongly decreased p120-Cas levels, and important structural changes consistent with impaired keratinocyte cell adhesion. Thus, unlike what has been proposed for oncogene-transformed or mitogenically stimulated cells, in differentiating keratinocytes tyrosine phosphorylation plays a positive role in control of cell adhesion, and this regulatory function appears to be important both in vitro and in vivo.

Plakophilin 1, a member of the armadillo multigene family, is a protein with dual localization in the nucleus and in desmosomes. To elucidate its role in desmosome assembly and regulation, we have analyzed its localization and binding partners in vivo. When overexpressed in HaCaT keratinocytes, plakophilin 1 localized to the nucleus and to desmosomes, and dramatically enhanced the recruitment of desmosomal proteins to the plasma membrane. This effect was mediated by plakophilin 1's head domain, which interacted with desmoglein 1, desmoplakin, and keratins in the yeast two-hybrid system. Overexpression of the armadillo repeat domain induced a striking dominant negative phenotype with the formation of filopodia and long cellular protrusions, where plakophilin 1 colocalized with actin filaments. This phenotype was strictly dependent on a conserved motif in the center of the armadillo repeat domain. Our results demonstrate that plakophilin 1 contains two functionally distinct domains: the head domain, which could play a role in organizing the desmosomal plaque in suprabasal cells, and the armadillo repeat domain, which might be involved in regulating the dynamics of the actin cytoskeleton.

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Abstract. Recombinant DNA technology has been used to analyze the first step in keratin intermediate filament (IF) assembly; i.e., the formation of the double stranded coiled coil. Keratins 8 and 18, lacking cysteine, were subjected to site specific in vitro mutagenesis to change one amino acid in the same relative position of the a-helical rod domain of both keratins to a cysteine. The mutations lie at position -36 of the rod in a "d" position of the heptad repeat pattern, and thus air oxidation can introduce a zerolength cystine cross-link. Mutant keratins 8 and 18 purified separately from Escherichia coli readily formed cystine homodimers in 2 M guanidine-HCl, and could be separated from the monomers by gel filtration. Heterodimers with a cystine cross-link were obtained when filaments formed by the two reduced monomers were allowed to oxidize. Subsequent ion exchange chromatography in 8.5 M urea showed that only a single dimer species had formed. Diagonal electrophoresis and reverse phase HPLC identified the dimer as the cystine containing heterodimer. This heterodimer readily assembled again into IF indistinguishable from those obtained from the nonmutant counterparts or from authentic keratins. In contrast, the mixture of cystine-stabilized homodimers formed only large aberrant aggregates. However, when a reducing agent was added, filaments formed again and yielded the heterodimer after oxidation. Thus, the obligatory heteropolymer step in keratin IF assembly seems to occur preferentially at the dimer level and not during tetramer formation.Our results also suggest that keratin I and II homodimers, once formed, are at least in 2 M guanidine-HC1 a metastable species as their mixtures convert spontaneously into heterodimers unless the homodimers are stabilized by the cystine cross-link. This previously unexpected property of homodimers explains major discrepancies in the literature on the keratin dimer.

An essential constituent of the integrated stress response (ISR) is a reversible translational suppression. This mRNA silencing occurs in distinct cytoplasmic foci called stress granules (SGs), which transiently associate with processing bodies (PBs), typically serving as mRNA decay centers. How mRNAs are protected from degradation in these structures remains elusive. We identify that Zipcode-binding protein 1 (ZBP1) regulates the cytoplasmic fate of specific mRNAs in nonstressed cells and is a key regulator of mRNA turnover during the ISR. ZBP1 association with target mRNAs in SGs was not essential for mRNA targeting to SGs. However, ZBP1 knockdown induced a selective destabilization of target mRNAs during the ISR, whereas forced expression increased mRNA stability. Our results indicate that although targeting of mRNAs to SGs is nonspecific, the stabilization of mRNAs during cellular stress requires specific protein–mRNA interactions. These retain mRNAs in SGs and prevent premature decay in PBs. Hence, mRNA-binding proteins are essential for translational adaptation during cellular stress by modulating mRNA turnover.

Plakophilins 1-3 are members of the p120(ctn) family of armadillo-related proteins. The plakophilins have been characterized as desmosomal proteins, whereas p120(ctn) and the closely related delta-catenin, ARVCF and p0071 associate with adherens junctions and play essential roles in stabilizing cadherin mediated adhesion. Recent evidence suggests that plakophilins are essential components of the desmosomal plaque where they interact with desmosomal cadherins as well as the cytoskeletal linker protein desmoplakin. Plakophilins stabilize desmosomal proteins at the plasma membrane and therefore may function in a manner similar to p120(ctn) in the adherens junctions. The three plakophilins reveal distinct expression patterns, and although partially redundant in their function, mediate distinct effects on desmosomal adhesion. Besides a structural role, a function in signaling has been postulated in analogy to other armadillo proteins such as beta-catenin. At least plakophilins 1 and 2 are also localized in the nucleus, and all three proteins occur in a cytoplasmic pool. This review aims to summarize the current knowledge of plakophilin function in the context of cell adhesion, signaling and their putative role in diseases.