GTP-binding proteins of the Rho/Rac family: regulation, effectors and functions in vivo

Abstract

Rho/Rac proteins constitute a subgroup of the Ras superfamily of GTP hydrolases. Although originally implicated in the control of cytoskeletal events, it is currently known that these GTPases coordinate diverse cellular functions, including cell polarity, vesicular trafficking, the cell cycle and transcriptomal dynamics. In this review, we will provide an overview on the recent advances in this field regarding the mechanism of regulation and signaling, and the roles in vivo of this important GTPase family.

Figure 1

Dendrogram showing the classification of Rho/Rac subfamily members according to structural similarity criteria. Members of each subfamily are highlighted using the same color code and grouped by shaded areas. The first symbol used for each GTPase corresponds to that approved by the Human Genome Organization Gene Nomenclature Committee. When appropriate, other commonly used names are also included. The same criterium has been followed in the rest of this review article.

Figure 2

Schematic representation of the biosynthesis (top), sequestration (middle) and regulatory (bottom) cycles of Rho/Rac proteins. In the latter case, we have included the prototypical GDP/GTP cycle as well as other regulatory steps mediated by the action of either effectors or other biological pathways (ubiquitination, protease cleavage, internalization). The main steps in each cycle are highlighted using dark-gray arrows. Other less common regulatory interactions are indicated in light-gray arrows (when resulting in an activation signal) or blunted lanes (when resulting in a downmodulation signal). The enzymes catalyzing those steps are shown in green. For the sake of simplicity, we have not included here other post-translational events of Rho/Rac proteins that have been described in the main text such as palmitoylation. It is also still unclear whether the insertion of the GTPase into the docking membrane is achieved when in the GDP or GTP-bound state. The latter case is not contemplated in the scheme and would require the activation of the GTPase by GEFs, the re-association of the GTP-bound GTPase with either RhoGDI or other carrier proteins, and the subsequent delivery of the GTPase to the target membrane. Abbreviations used are: CAAX, an acronym derived from the combination of C=cysteine, A=aliphatic amino acids and X=Met, Ser, Ala or Gln; Cyt, cytosol; EM, endomembranes; ER, endoplasmic reticulum; FT, farnesyl transferase; GGT, geranyl-geranyl transferase; PM, plasma membrane; PRR BP, proline rich region binding protein. Consult main text for further details.

Figure 3

Structural determinants for the interaction of Rho/Rac proteins with downstream effectors. A: Scheme showing the residues of the switch (F37 and Y40) and α3′ regions involved in the selective interaction of GTP-bound Rac1 with effectors. Similar data have been obtained with other GTPases, including RhoA, RhoG and Cdc42. B: Crystal structure of GTP-bound Cdc42 associated to the Cdc42-binding region of Was. The reader can observe the extensive contacts established by Was with the switch I, switch II, β2 sheet and α5 helix of the GTPase.

Figure 4

Examples of the types of signaling outputs derived from the interaction of Rho/Rac subfamily proteins and effectors. See main text for further details. I, inhibitor.

Figure 5

Representation of the main developmental routes for hematopoietic cells and the steps that are dependent on either Rho/Rac subfamily proteins or Rho/Rac effectors. The GTPases and/or effectors involved in those steps are highlighted in blue. The processes impaired by the gene inactivations in each hematopoietic lineage are summarized into light-brown boxes.