Rho GDI constitutes a family with three mammalian members: RhoGDI alpha, the ubiquitously expressed archetypal member of the family; Ly/D4-GDI or RhoGDI beta, which has haematopoietic tissue-specific expression, particularly in B- and T-lymphocytes; and RhoGDI-3 or -gamma, which is membrane-anchored through an amphipathic helix and is preferentially expressed in brain, pancreas, lung, kidney and testis (1). RhoGDIs function by extracting Rho family GTPases including RhoA, Rac1, and Cdc42 from membranes and solubilizing them in the cytosol. Moreover, both in vitro and in vivo they interact only with prenylated Rho proteins. They also inhibit nucleotide exchange and GTP hydrolyzing activities on Rho proteins by interacting with their switch regions and probably restricting accessibility to GEFs and GAPs (2). The N-terminal domain of RhoGDI alpha binds to the switch region of the GTPases, affecting the GDP-GTP cycling, whereas the C-terminus accommodates the isoprenyl moiety of the GTPases in its hydrophobic pocket, regulating cytosol/membrane partitioning (3). RhoGDI alpha can be phosphorylated on two sites, Ser101 and Ser174, by PAK (P21-activated kinase) leading to selective activation of Rac1, but not RhoA or Cdc42, both in vitro and in vivo (4).
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