Protein Modules in Cellular Signalling


Heilmeyer, L.,
Friedrich, P.

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Dynamic stability of cells is a function of coordination and counterbalance between intracellular signalling events. Therefore, the knowledge of those molecules which form signalling cascades or signalling modules is of prime importance in understanding living processes. Signalling proteins, the component members of different cascades, are now the focus of intense interest. Protein phosphorylation dephosphorylation is the prevalent mechanism by which signalling molecules transduce their signals. Therefore, it seems timely to review in a NATO Advanced Study Institute those signalling molecules that are involved in the formation of protein kinase modules or more generally in protein complexes like "signallosomes". It is indeed clear that our understanding of such complex structure/function relationships will require a knowledge of the three dimensional structure of the participating protein components. Thus, for example the process of ligand binding, the association of proteins via specific interacting domains, the catalytic mechanism of protein kinases and phosphatases must be described at the atomic level. Likewise an understanding of the temporal and spatial relationship between different signalling molecules within the cellular environment will be essential to our comprehension of cellular regulation and, in particular, how normal cells can undergo transformation. Within the context of the Advanced Study Institute, therefore, techniques to study these molecular mechanisms will be reviewed. This will comprise: a) an analysis of structures by X-ray crystallography; the complex situation of a signalling protein within a cell must be looked at for example by b) the manipulation of protein kinase and phosphatase activities within a cell e.g. by micro injection of inhibitors or activators or by overexpression of these enzymes; c) techniques that can reveal dynamic processes i.e. movement of proteins and finally, d), characterization of proteins at a single molecule level for example in optical traps.
Protein kinases will be described in much detail are and, specifically, the catalytic domains of growth factor receptors. These molecules are located at point of conversion or diversion in regulatory networks. One of the goals of this Advanced Study Institute will be to examine the coordination of such activities within signalling modules connecting points of junction. Other major players are protein phosphatases which are directed by targeting subunits to specialised locations to keep balance in signalling modules. An exploding area is concerned with lipid metabolism in plasma membranes. Families of lipid kinases like those of the phosphatidylinositol 3-kinases, the phosphatidylinositol 4-kinases, and the phosphatidylinositolphosphate kinases have been recently described which provide a much more detailed picture of lipid signalling as previously thought. An important new aspect is the involvement of these signalling lipids in vesicular transport. It opens the view that these signalling systems participate in a network causing intracellular calcium release, induction of movement, and vesicular transport. It seems, therefore, that an integrated discussion of these diverse phenomena in a NATO Advanced Study Institute will generate a far broader understanding of protein phosphorylation as a major determinant in cellular homeostasis and thus human health and disease.