Abstract The small Rho GTPases are a family of signalling proteins that act as molecular switches to regulate a number of key cellular processes, including actin remodelling, cell adhesion, cell polarity and cell migration. Recently, they were also found to play vital roles in nervous system development, immune system development and angiogenesis. The human family of Rho GTPases contains 20 members; however, only three of them; RhoA, Racl and Cdc42, have been studied in detail. Solving the pathways controlled by the other members of this important signalling protein family is clearly essential to our understanding of the actin cytoskeleton. The research within this thesis focused on the characterization of the signalling pathways of one novel and poorly-defined Rho GTPase: Rif. The Rif GTPase is a relatively recent addition to the Rho family; it is found only in chordates and displays a relatively low homology to other family members. Activated Rif was found to be an alternative trigger for the formation of actin stress fibers in epithelial cells through effector mDial. Unlike the classical stress fiber inducer RhoA, Rif does not raise ROCK activity in cells, instead Rif appears to regulate the localization of myosin light chain phosphorylation. This study establishes Rif as a general regulator of Diaphanous-related formins and shows how non-classical Rho family members can access classical Rho pathways to create new signalling interfaces in cytoskeletal regulation. Proteomics were carried out in this thesis to identify potential binding partners of Rif. Through these studies I have shown that Rif interacts with FARPl, which is a RhoA GEF; and plexinA4, which is a developing nervous system guidance cue receptor. PlexinA4, FARPl and Rif form a heterotrimer at the cell membrane. At low local expression levels, the priority for Rif is to bind the C-terminus of FARPl and enhances its RhoGEF activity. In contrast, at higher local expression levels, Rif competes with the FERM domain ofFARPl for plexinA4 binding and forms a feedback loop to prevent overactivation of plexinA4/FARPl signalling pathway. These results suggest that Rif can not only work as a GTPase to remodel actin cytoskeleton, but also act as a regulator to regulate the activity of the other small Rho GTPases.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:555617 |
| Date | January 2011 |
| Creators | Fan, Lifei |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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