Browsing by Subject "rac1 GTP-Binding Protein"
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Item Hyperactive Rac1 Drives MAPK-Independent Proliferation in Melanoma by Assembly of a Mechanosensitive Dendritic Actin Network(2018-06-26) Mohan, Ashwathi; Brekken, Rolf A.; Danuser, Gaudenz; Cobb, Melanie H.; Alto, NealCancer cells use a variety of mechanisms to subvert growth regulation and overcome environmental challenges. Often, these same mechanisms enable cancer cells to develop resistance to targeted therapies. Here, we describe how a hyperactivating mutation of the Rac1 GTPase (Rac1P29S) harnesses Rac1's function as a regulator of actin polymer assembly to sustain cell cycle progression in growth limiting conditions. This proliferative advantage supports metastatic colonization of melanoma cells and confers insensitivity to inhibitors of the mitogen-activated protein kinase (MAPK) pathway, a frequent target for melanoma treatment. Rac1P29S bypasses the MAPK axis through a mechanism that necessitates cell-matrix attachment, however, does not depend on integrin-mediated focal adhesion assembly and focal adhesion kinase signaling. Even without involvement of canonical adhesion signaling, cells carrying the Rac1P29S mutation show elevated traction upon drug treatment and require mechanical resistance from their surrounding matrix to gain a proliferative advantage. We describe an alternative arm for cell mechanosensing, whereby actin polymerization against a matrix of minimal rigidity organizes biochemical cues to drive proliferative signals. Hyperactivation of Rac1 by the P29S mutation channels this pathway in melanoma through Arp 2/3-dependent formation of a constrained actin brush network that results in the inactivation of tumor suppressor NF2/Merlin. These data suggest an alternative mechanism for mechanosensitive growth regulation that can be hijacked by cancer cells to circumvent the adverse conditions of foreign microenvironments or drug treatment.Item Regulation of p190RhoGEF by Activated Rho and Rac GTPases: Amplification and Crosstalk(2017-04-14) Dada, Olugbenga Adesola; Taussig, Ronald; Alto, Neal; Yu, Hongtao; Sternweis, Paul C.The Rho family of monomeric GTPases regulates a wide range of cellular processes including cytoskeletal structure, motility, cell division, gene transcription, vesicular transport, and various enzymatic activities. Activation of Rho proteins largely depends on Rho Guanine nucleotide Exchange Factors (RhoGEFs), which catalyze the exchange of GTP for GDP on Rho. For classical RhoGEFs, the exchange activity resides in a Dbl homology (DH) domain, which is linked to a pleckstrin homology (PH) domain that subserves various functions. We have crystallized and solved structures of the PH domain from p190RhoGEF bound to either RhoA•GTP or Rac1•GTP. The interfaces between activated Rac1 or RhoA with the PH domain utilize the same hydrophobic surface on the PH domain. Similar to RhoA, activated Rac1 interacts with the PH domain via its effector-binding surface albeit burying less surface area than the interaction of RhoA with the PH domain. Consequently, the PH domain of p190RhoGEF has a higher affinity for RhoA than Rac1. Both activated RhoA and Rac1 can stimulate exchange of nucleotide on RhoA by localization of the p190RhoGEF to the substrate, RhoA•GDP, in vitro; mutations of key hydrophobic residues in the PH domain abolish this stimulation. Among members of the homologous Lbc subfamily of RhoGEFs, p190RhoGEF exhibited the greatest capacity for regulation by Rac1. While interaction of RhoA with the PH domain provides a mechanism for direct positive feedback, the novel interaction of activated Rac1 with the PH domain of p190RhoGEF reveals a potential mechanism for cross-talk regulation between the Rho and the Rac signaling pathways.