Signal transduction towards the cell nucleus
Freien Universität Berlin, Ph.D. thesis, Supervisor: Prof. F. Hucho
Cells communicate with their environment by employing receptor proteins, which except of a few exceptions reside in the plasma membrane. These receptors scan the cellular environment for signal molecules, ligands that activate the receptor upon binding and transmit a signal to the cytosolic face of the membrane. Here, often GTP-binding proteins pick up the signal and convert it in the activity change of effector molecules.
Protein kinase C stands for a family of protein kinases that belong to the second-messenger activated protein kinases, which form the entry point of an integrative layer of kinases and phosphatases making sense of the different signals a cell encounters. The classical forms of PKC, isoforms alpha, beta and gamma, are translocated to membranes upon binding of calcium to their C2-domain. Activation then occurs at the membrane by diacylglycerol binding, which is initially created from phosphatidyl-inositol-4.5-bisphosphate by a phospholipase C activated either via a G protein-coupled receptor or a receptor tyrosine kinase (PI-response).
At the time I started my Ph.D.-thesis, it had been observed that PKC could stimulate cell proliferation. Apparently, signals passing PKC were transmitted into the nucleus by this process. One possibility seemed to be a translocation of PKC to the cell nucleus and an activation at the nuclear membranes. It seemed conceivable that signalling procedures at the nuclear membranes might exist in analogy to the mechanisms at the plasma membrane.
I therefore established procedures to purify and to subfractionate nuclei from tissues (brain, liver) and from cultured cells. By applying protein chemical methods on purified nuclear envelopes, I identified a permanently active, membrane-associated form of classical protein-kinase-C isoform firmly attached to the nuclear envelope (Buchner et al. (1992)). Photoaffinity labelling with radioactively-labelled GTP produced evidence for a small, monomeric G protein associated with the nuclear envelope (Otto et al. (1992)), probably the G-protein „ran“ controlling the nucleocytoplasmic transport, which at that time could not be identified.
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