Abstract
The NF-2 gene (neurofibromatosis type 2) encodes for the tumour suppressor protein merlin, which is related to ERM proteins (ezrin, radixin, moesin). Merlin deficiency leads to uncontrolled growth and an aberrant actin cytoskeleton. Merlin acts as a tumour suppressor by activating the contact inhibition of growth and interfering with Ras signal transduction. This tumour suppressor function is activated through high cell density or the cell-matrix component hyaluronic acid and depends on merlin dephosphorylation on serine-518. In this thesis it was possible to show that dephosphorylation on serine-518 is not the only essential modification for tumour suppression. Additionally dephosphorylation on threonine-272 and phosphorylation on threonine-576 inhibits transformation. Short-term activation of merlin via hyaluronic acid is accompanied by changes in the actin cytoskeleton and a merlin-mediated inhibition of the small G-protein Rho. The dynamics of this inhibition points towards an involvement of a RhoGAP. Merlin indeed immunoprecipitates and colocalizes with p190RhoGAP. Moreover this interaction is specific for the active merlin mutants S518A (mimics dephosphorylation) and T576D (pseudo-phosphorylated). p120RasGAP a known binding partner of p190RhoGAP binds as well to merlin. This suggested that the p120RasGAP substrate Ras could be a target for merlin-mediated inhibition after hyaluronic acid incubation. This is in fact the case. Conversely loss of merlin by the use of RNAi results in activation of Ras. This interaction of merlin with p190RhoGAP and p120RasGAP regulated via differential phosphorylation could be the reason for the inhibition of the small G-proteins Rho and Ras and correlates with the tumour suppressor activity. Furthermore these new binding partners of merlin could allow an influence on proliferation, cell adhesion and organization of the cytoskeleton.
Published Version
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