Ras acylation, compartmentalization and signaling nanoclusters (Review)

Abstract

Ras proteins have become paradigms for isoform- and compartment-specific signaling. Recent work has shown that Ras isoforms are differentially distributed within cell surface signaling nanoclusters and on endomembranous compartments. The critical feature regulating Ras protein localization and isoform-specific functions is the C-terminal hypervariable region (HVR). In this review we discuss the differential post-translational modifications and reversible targeting functions of Ras isoform HVR motifs. We describe how compartmentalized Ras signaling has specific functional consequences and how cell surface signaling nanoclusters generate precise signaling outputs.

Figure 1

The Ras Hypervariable Region (HVR). (A) Ras isoform C-terminal HVRs have different combinations of post-translational lipid modifications and membrane interacting polybasic motifs that specify differential trafficking and localization. (B) Ras isoform localization is dynamic; changes in H- and N-Ras activation state or palmitoylation alter the association with cell surface subdomains/clusters and endomembranous compartments.

Figure 2

Plasma membrane Ras nanocluster parameters. EM imaging of immunogold-labeled H-Ras molecules on 2-D plasma membrane sheets (A); bar =50 nm. Ras isoforms dynamically localize to distinct signaling nanoclusters with differential cholesterol dependence (B). Other characteristics of Ras nanoclusters obtained from EM and advanced light microscopy studies are summarized in (C). This Figure is reproduced in color in Molecular Membrane Biology online.

Figure 3

Molecular Dynamics simulations of H-Ras HVR interactions with the plasma membrane. The H-Ras G-domain and HVR lipid moieties adopt different orientations with respect to the plane of the membrane when GTP-bound (A) and GDP-bound (B). The GTP-bound conformation is stabilized by membrane contacts with basic residues (R128 and R135) on helix α4. These contacts are lost in GDP-H-Ras, which is stabilized by an alternative set of basic residues within the HVR. Note also that the palmitoyl groups exhibit a more extended conformation when H-Ras is GDP-bound. Phosphorous atoms of lipid head groups of the inner membrane leaflet are shown in grey and H-Ras lipid anchors are in light blue. Important basic residues in H-Ras are shown in dark blue and acidic residues in red. Reproduced with permission from [72].

Figure 4

Analog-digital coupling involving Ras signaling nanoclusters. (A) Analog signals allow graded responses proportional to input whereas digital or high gain switch-like signaling results in maximal output from a wide range of inputs – amplifying weak inputs into maximum outputs. (B) Ras nanoclusters operate as digital amplifiers. (C) Digital quantal sampling of analog inputs can generate high fidelity analog-like outputs if sampling time (Ras activation/de-activation and nanocluster lifetime) is very fast and sample precision (number of nanoclusters) is high. (D) High fidelity digital Ras nanocluster signaling ensures that the analog extracellular signal is converted into a graded cytosolic signaling response. This Figure is reproduced in color in Molecular Membrane Biology online.