Members of the Arf Family as Triggers for Coat Formation
The formation of intracellular transport carriers is frequently supported by small GTPases, for instance by members of the Arf family of GTPases. Arf proteins possess an aminoterminal amphipathic helix, which upon activation of the GTPase dives into the membrane, thus triggering an asymmetric expansion of one leaflet of the lipid bilayer and concomitantly the generation of membrane curvature (Krauss et al., 2009).
Figure 1: Arf1 encodes an amphipathic helix at its N-terminus (left: helical wheel representation of the first 14 amino acids), which is exposed upon GTP-loading. Note that the Gly residue at position 2 is myristoylated and that this modification increases the hydrophobicity of one side of the helix. Upon addition to liposomes recombinant Arf1-GTP binds to and deforms the membrane into tubular structures as indicated by EM micrographs (right; in collaboration with Aurélien Roux, Institut Curie, Paris). Mutation of key residues (indicated by a red circle) to charged amino acids completely abolishes the membrane deforming activity of Arf1.
In addition, some members interact with and activate phosphoinositide-metabolizing enzymes, as we have previously demonstrated for Arf6. Arf6-GTP thereby initiates the generation of a pool of PI(4,5)P2, which greatly enhances the affinity of endocytic adaptor proteins (e.g. AP180 and AP-2) for neuron-derived membrane surfaces (Krauss et al., 2003). Furthermore, several Arf GTPases directly associate with cargo-recognizing adaptor proteins and thereby trigger their membrane translocation.
Figure 2: In living cells overexpression of Arf1-eGFP induces the formation of tubular structures emanating from the Golgi apparatus. The mutant F5W increases the hydrophobicity of the N-terminal helix and thereby strongly enhances tubule formation. The tubules also stain positive for Arf-dependent adapter proteins (in this example the PI(4)P-recognizing PH-domain of FAPP1).