Dual‐Specificity Ras/Rap GTPase Activating Proteins


The small GTP‐binding proteins Ras and Rap regulate important cellular events switching between an activated GTP‐bound form and an inactivated GDP‐bound state. Guanine exchange factors (GEFs) control the GDP to GTP‐bound cycle, while the conversion, through hydrolysis, of GTP to GDP is catalysed by GTPase‐activating proteins (GAPs). The GAPs complete Ras and Rap catalytic site for efficient GTP hydrolysis. Although Ras and Rap are highly homologous, they possess different residues in the catalytic site. In consequence, Ras and Rap GTPase‐activating proteins (RasGAPs and RapGAPs) are structurally unrelated and use different mechanisms. Surprisingly, there are several RasGAPs with dual Ras/Rap specificity: SynGAP; GAP1 family members GAP1IP4BP, Rasal and Capri; and Plexins. This characteristic was an intriguing issue for years, but today structural and biochemical studies have enlightened the overall general mechanism of these dual GAPs.

Key Concepts

  • Small GTP‐binding proteins Rap and Ras control different cellular signalling switching between inactive GDP and active GTP‐bound states.
  • Ras and Rap GTPase‐activating proteins (RasGAPs and RapGAPs) inactivate their downstream signalling catalysing the GTP hydrolysis, using RasGAP and RapGAP domains.
  • RasGAPs–Ras‐GTP interaction positions Ras Gln61, situated in switch II loop, for nucleophilic attack, while the GAP contributes an Arg (arginine finger) to neutralise unfavourable negative charges of the reaction intermediate.
  • Rap has a Thr in position 61, and RapGAPs catalyse GTP hydrolysis, contributing an Asn (Asn thumb) for nucleophilic attack.
  • There are five RasGAPs with dual Rap/Ras specificity: SynGAP, Rasal, GAP1IP4BP, Capri and Plexin.
  • Dual GAPs need extra‐GAP domains to act as RapGAPs.
  • Dual GAPs promote a specific orientation of Rap switch II, locating Gln63 as the catalytic residue.
  • Plexin‐Rap X‐ray structure showed that residues of GAP domain and an extra‐GAP motif (juxtamembrane segment) are responsible for Gln63 correct orientation.
  • SynGAP, Rasal, GAP1IP4BP, Plexin and Capri do not share the same residues, and thus they still need to be found.

Keywords: Ras; Rap; GTPase‐activating protein; dual‐specificity GAP; SynGAP; Rasal; GAP1IP4BP; Capri; Plexin

Figure 1. (a) Crystallographic structure of p120–Ras–GDP–AlFx complex (pdb 1WQ1) showing Ras in yellowish green and GAP domain in orange. A zoomed view of the catalytic site shows the residues implicated in GTP hydrolysis. (b) Crystallographic structure of the complex between the GAP domain of RapGAP1 (orange) and Rap–GDP–BeFx (yellow) (pdb 3BRW). A zoomed view of the catalytic site shows the residues implicated in GTP hydrolysis.
Figure 2. Domains and regions of human dual‐specificity Ras/RapGAPs.
Figure 3. Structure of dual‐specificity Ras/RapGAPs. (a) Model of SynGAP–Rap complex based on SynGAP C2‐GAP crystallographic structure (pdb 3BXJ). Rap in green, GAP domain in red, C2 in yellow and Rap switch II highlighted in dark blue. (b) Rasal 3D reconstruction by electron microscopy, with homologous 3D structures of the distinct domains docked inside. C2A: yellow, C2B: orange, GAP: red, PH: blue and Ct: purple. Cuellar, https://www.degruyter.com/view/j/bchm.2018.399.issue‐1/hsz‐2017‐0159/hsz‐2017‐0159.xml. (c) Crystallographic structure of PlexinC1–Rap complex (pdb 4M8N). Rap in green, GAP domain in red, juxtamembrane segment in yellow and Rap switch II highlighted in dark blue. (d) A zoomed view of the catalytic site shows the Rap and Plexin residues implicated in switch II orientation.
Figure 4. Model of common dual RAs/Rap GAP activity. Rap switch II interacts with an α‐helix of the GAP domain (α‐helix 6 of the GAP domain) and the extra‐GAP domains to achieve the catalytic orientation, with Q63 stabilising the hydrophilic water molecule for GTP hydrolysis. The GAP arginine finger completes the catalytic site neutralising negative charges at GTP γ‐phosphate.


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Further Reading

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Sot, Begoña(Oct 2018) Dual‐Specificity Ras/Rap GTPase Activating Proteins. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0028180]