under the auspices of the Japanese-German Cooperative Program in Cancer Research, the third of the biennial Japanese-German Workshops on "Molecular and Cellular Aspects of Carcinogenesis" was held from 24 to 26 October, 1991. Reports by 21 researchers from both countries focused on structural and functional properties of cytoskeletal components and extracellular matrices, molecular mechanisms in signal transduction and malignant transformation, and molecules involved in cell adhesion, invasiveness, and metastasis.
Antibodies specifically recognizing phosphorylated sites of glial fibrillary acidic protein were developed by Dr. M. Inagaki (Nagoya). These antibodies were used to elucidate regulatory mechanisms governing the assembly and disassembly of glial filaments. By immunofluorescence analysis and immunoblotting, it was shown that the antibodies specifically bind to glial filaments of mitotic astrocytes, supporting the notion that increased phosphorylation may directly influence the structural organization of glial filaments during mitosis. Dr. W.W. Franke (Heidelberg) reported on cytokeratins (CKs), i.e., cytoskeletal proteins forming intermediate-sized filaments (IFs) and specific junction proteins (forming desmosomes or hemi-desmosomes) synthesized in differentiating normal epithelia and in epithelial cell-derived tumors. CKs represent a multigene family of 20 epithelial and 10 trichocytic polypeptides expressed in epithelial tissues in different pathway-specific combinations. Bundles of IFs are anchored at the cytoplasmic plaques of desmosomes, structures integrating cytoplasmic proteins .(plakoglobins, desmoplakin, "band-VI protein") and the carboy-terminal extensions of certain trans-membrane proteins, Le., desmosomespecific members of the cadherin family of adhesion molecules (desmogleins, desmocollins) involved in homotypic and probably homophilic cell-cell interactions. Unexpected complexities and tissue-specific differences of desmogleins and desmocollins have recently been detected with the use of cDNA techniques. Thus, a given cell of a stratified epithelium may contain at least 6 different molecules of the cadherin family.
The potential role of membrane-bound and cytosolic inositol phosphates in cytoskeletal and chromatin rearrangements was discussed by Dr. G.W. Mayr (Bochum), on the basis of recently demonstrated interactions of phosphatidylinositol 43bisphosphate [PtdIns(4,5)P2] with the actin-binding proteins gelsolin and profilin. The identification of nuclear receptors for inositol 1,4,5-triphosphate [Ins( 1,4,5)P,] and the [Ins( 1,4,5)P3]-sensitive release of Caz+ from isolated nuclei suggest the involvement of inositol phosphates in nuclear function(s). Direct interaction of water-soluble inositide head groups with monomeric and filamentous actin was investigated, as well as the role of inositol phosphates other than Ins(l,4,5)P3 in nuclei and nuclear structures. The known phosphate-binding site of actin binds the sugar bisphosphates Fru(l,6)P2 and Ins( 1,4)P,, and the head groups of PtdIns4P and GroPIns4P, with much higher affinity than phosphate. Phosphotyrosine, too, is bound with high affinity. All of these phosphocompounds bind to filamentous actin better than to monomeric actin, inducing filarhent elongation. A highly significant correlation was found between the masses of Ins( 1,3,4,5,6)P, and InsP, and the nucleus-to-cell-volume ratios of a large number of animal tissues and cells. Isolated liver nuclei contain at least 2 types (high and low affinity) of InsP,-receptors. Ins( 1,3,4,5,6)P, antagonizes InsP, binding most efficiently. In nuclear extracts (,H)InsP, binds predominantly to histones. Binding to chromatin structures was demonstrated in polytene chromosomes from Chironornus thummi salivary glands (most pronounced in bands corresponding to active genes and in centromeric regions). Ongoing studies are focusing on the role of InsP,, and possibly Ins(1,3,4,5,6)P5, in chromosome structure and function. Dr. K. Mikoshiba (Osaka) and his coworkers have purified the Ins(1,4,5)P3 receptor (InsP,-R) and cloned its cDNA. They found InsP,-R to be identical to the P, , protein which is enriched in cerebellar Purkinje cells. The receptor is expressed in cells transfected with the InsP,-R gene, exhibits a high affinity for Ins(1,4,5)P3, and releases Ca2+ from membrane vesicles. While InsP,-R is abundant in Purkinje cells, the receptor is ubiquitously expressed in a variety of tissues. TnsP,-R is a glycoprotein forming a homotetramer, and is highly concentrated on smooth-surfaced endoplasmic reticulum. Novel sub-types of InsP,-R are expressed in a cell-type-and developmental-stagespecific manner.
Small GTP-binding proteins (G-proteins, smg) and their possible functions in signal transduction, gene expression and transformation were studied by Dr. K. Kaibuchi (Kobe). GDP-bound inactive forms and GTP-bound active forms of G-proteins are interconvertible by GDP/GTP exchange and GTPasc reactions. Both the former and the latter reactions are regulated by GDP/GTP exchange proteins (GEPs) and GTPase-activating proteins (GAPS). Two types of GEP were isolated: GDP dissociation stimulator (GDS) and GDP dissociation inhibitor (GDI). For the action of smg GDS, post-translational processing of the C-terminal region of its substrate srng is essential. smg GDS promotes the GDP/GTP exchange reaction of Ki-ras p21, which eventually leads to the transformation of NIH73T3 cells and to stimulation of the cfos promoter/enhancer, suggesting a regulatory role of smg GDS for Ki-ras p21. A. Wittinghofer (Heidelbpg) has analyzed the structure of the guanine nucleotide binding domain of ras" p21 in the triphosphate conformation at 1.4 A resolution. All binding interactions bctween the protein and GppNp and Mgz+ were identified in detail, as was the site of interaction between p21 and the effector, i.e., the GTPase-activating protein (GAP). The flexiblc rcgion around amino acids 61-65 exists in 2 conformations, one of which seems to be important for catalysis. A mechanism proposed for GTP hydrolysis involves glutamine-61 as a critical residue for activation of the nucleophilic reaction. Specific structural changes due to GTP hydrolysis are associated with a switch of the protein from the