Intermediate filaments as dynamic structures

Mitotic reorganization of the intermediate filament protein nestin involves phosphorylation by cdc2 kinase

The intermediate filament protein nestin is expressed during early stages of development in the central nervous system and in muscle tissues. Nestin expression is associated with morphologically dynamic cells, such as dividing and migrating cells. However, little is known about regulation of nestin during these cellular processes. We have characterized the phosphorylation-based regulation of nestin during different stages of the cell cycle in a neuronal progenitor cell line, ST15A. Confocal microscopy of nestin organization and (32)P in vivo labeling studies show that the mitotic reorganization of nestin is accompanied by elevated phosphorylation of nestin. The phosphorylation-induced alterations in nestin organization during mitosis in ST15A cells are associated with partial disassembly of nestin filaments. Comparative in vitro and in vivo phosphorylation studies identified cdc2 as the primary mitotic kinase and Thr(316) as a cdc2-specific phosphorylation site on nestin. We generated a phosphospecific nestin antibody recognizing the phosphorylated form of this site. By using this antibody we observed that nestin shows constitutive phosphorylation at Thr(316), which is increased during mitosis. This study shows that nestin is reorganized during mitosis and that cdc2-mediated phosphorylation is an important regulator of nestin organization and dynamics during mitosis.

Apoptosis-induced cleavage of keratin 15 and keratin 17 in a human breast epithelial cell line

Keratin 15 (K15) and keratin 17 (K17) are intermediate filament (IF) type I proteins that are responsible for the mechanical integrity of epithelial cells. By analyzing the human breast epithelial cell line H184A1 before and after induction of apoptosis by high-resolution two-dimensional gel electrophoresis (2-DE) we identified the caspase-mediated cleavage of keratins 15 and 17. After induction of apoptosis three fragments of both K15 and K17 could be observed by 2 -DE. K15 and K17 proteolysis was observed during staurosporine-induced apoptosis and anoikis (anchorage-dependent apoptosis) as well and was shown to be caspase-dependent. By using mass spectrometry we could determine the caspase cleavage sites, one in K15 and two in K17. The sequence VEMD/A at the cleavage site located in the conserved linker region was found in K15 and K17. A further cleavage site was identified in the tail region of K17 with the recognition motif EVQD/G.

Type I and type II cytokeratin cDNAs from the zebrafish (Danio rerio) and expression patterns during early development

Full-length cDNAs of a type I (zfCKI), and a type II (zfCKII) cytokeratin from the adult zebrafish, Danio rerio, were characterized and their expressions studied during early development and in the adult. The 1,426 bp long zfCKI cDNA encodes a 46.7 kD protein, whereas the 2,398 bp zfCKII cDNA encodes a protein of 58.6 kD. zfCKI and zfCKII each have a central rod domain that is characteristic of intermediate filaments and which share 73%-91% and 87%-93% similarity, respectively, with those of type I and type II cytokeratins from zebrafish, goldfish, and the rainbow trout. The central rod domains of zfCKI and zfCKII also contain the IF signature motif, IA[T/E]YR[K/R]LL[D/E]. zfCKI has, in addition, a leucine-zipper motif at a.a. residues 184-205 and 191-212. Both zfCKI and zfCKII mRNAs are expressed in the epidermis of the zebrafish. zfCKII mRNA was both maternally inherited and zygotically transcribed and was detected from the one-cell embryo to adult stages. zfCKII was also strongly expressed specifically during the 20-somites, protruding-mouth, and adult stages. In the adult, it was uniformly expressed in the skin, fins and scale epidermis. In contrast, zfCKI mRNA was undetectable in the oocyte but was zygotically transcribed from the epiboly stage onwards. Its expression in the skin was strong only up to the swimming larva stage and was weak and patchy in the adult. Both zfCKI and zfCKII were expressed in the neurons and glial cells of the brain and spinal cord. In the adult eye, zfCKI and zfCKII were expressed in the ganglion cell layer and the retina, but zfCKII was also strongly expressed in the cornea as well as in chondrocytes in the skull.

Role of cytoskeleton in apoptosis

Apoptosis is a form of cell death that takes place under physiologic conditions, and plays a key role in the control of biological processes such as embryonic development, tissue remodelation and renewal, or regulation of cell populations. Since its discovery in the early 1970s, there have been many relevant advances in the knowledge of the biochemical and molecular events involved in apoptosis. However, although the apoptotic process was defined on the basis of morphologic observations, only recently have we started to elucidate the molecular mechanisms that drive the structural changes observed in cells undergoing apoptosis. The article reviews current knowledge about the implications of cytoskeleton components (microfilaments, intermediate filaments, microtubules, and other cytoskeleton-related proteins) in the dynamics of apoptosis.

The RNA polymerase II core subunit 11 interacts with keratin 19, a component of the intermediate filament proteins

We have previously cloned the human RNA polymerase II subunit 11, as a doxorubicin sensitive gene product. We suggested multiple tasks for this subunit, including structural and regulatory roles. With the aim to clarify the human RNA polymerase II subunit 11 function, we have identified its interacting protein partners using the yeast two-hybrid system. Here, we show that human RNA polymerase II subunit 11 specifically binds keratin 19, a component of the intermediate filament protein family, which is expressed in a tissue and differentiation-specific manner. In particular, keratin 19 is a part of the nuclear matrix intermediate filaments. We provide evidence that human RNA polymerase II subunit 11 interacts with keratin 19 via its N-terminal alpha motif, the same motif necessary for its interaction with the human RNA polymerase II core subunit 3. We found that keratin 19 contains two putative leucine zipper domains sharing peculiar homology with the alpha motif of human RNA polymerase II subunit 3. Finally, we demonstrate that keratin 19 can compete for binding human RNA polymerase II subunit 11/human RNA polymerase II subunit 3 in vitro, suggesting a possible regulatory role for this molecule in RNA polymerase II assembly/activity.

Characterization of the chicken transitin gene reveals a strong relationship to the nestin intermediate filament class

Our laboratory previously reported that transitin is a radial glial intermediate filament protein sharing the basic structural features common to all intermediate filament (IF) proteins. It contains an alpha-helical core domain flanked by a short nonhelical head and a long COOH-terminal tail. The core sequence of transitin shows the greatest similarity to Xenopus tanabin and to rat and human nestin. We also reported that transitin has multiple splice variants derived from the deletion or inclusion of a leucine-zipper heptad repeat domain in the COOH-terminal tail. In the present study, we provide new evidence to support the classification of nestin and transitin in the same group of IF proteins based on the number and position of its introns. In addition, we suggest that the different isoforms of transitin are produced by a splicing mechanism that recognizes consensus 5' and 3' splice sites contained within the coding sequence of the leucine-zipper heptad repeat domain.

Estrogen regulates the association of intermediate filament proteins with nuclear DNA in human breast cancer cells

In a previous study we showed that the levels of the intermediate filament proteins, cytokeratins 8, 18, and 19, in the nuclear matrix-intermediate filament (NM-IF) fraction from the hormone-dependent and estrogen receptor (ER)-positive human breast cancer cell line T-47D5 were regulated by estrogens. In contrast, estrogens did not regulate the cytokeratins in the NM-IF fraction of the hormone-independent and ER-positive cell line, T5-PRF. In this study, human breast cancer cells were treated with cis-diamminedichloroplatinum to cross-link protein to nuclear DNA in situ, and proteins bound to DNA were isolated. We show that cytokeratins 8, 18, and 19 of T-47D5 and T5-PRF were associated with nuclear DNA in situ. The levels of the cytokeratins 8, 18, and 19 bound to nuclear DNA or associated with the cytoskeleton of T-47D5 human breast cancer cells decreased when estrogens were depleted or the pure antiestrogen ICI 164,384 was added. In contrast, the cytokeratin levels associated with nuclear DNA or cytoskeleton were not significantly affected by estrogen withdrawal or antiestrogen administration in T5-PRF cells. These observations suggest that estrogen regulates the organization of nuclear DNA by rearrangement of the cytokeratin filament network in hormone-dependent, ER-positive human breast cancer cells and that this regulation is lost in hormone-independent, ER-positive breast cancer cells.