Ras, an actor on many stages: posttranslational modifications, localization, and site-specified events

Abstract

Among the wealth of information that we have gathered about Ras in the past decade, the introduction of the concept of space in the field has constituted a major revolution that has enabled many pieces of the Ras puzzle to fall into place. In the early days, it was believed that Ras functioned exclusively at the plasma membrane. Today, we know that within the plasma membrane, the 3 Ras isoforms-H-Ras, K-Ras, and N-Ras-occupy different microdomains and that these isoforms are also present and active in endomembranes. We have also discovered that Ras proteins are not statically associated with these localizations; instead, they traffic dynamically between compartments. And we have learned that at these localizations, Ras is under site-specific regulatory mechanisms, distinctively engaging effector pathways and switching on diverse genetic programs to generate different biological responses. All of these processes are possible in great part due to the posttranslational modifications whereby Ras proteins bind to membranes and to regulatory events such as phosphorylation and ubiquitination that Ras is subject to. As such, space and these control mechanisms act in conjunction to endow Ras signals with an enormous signal variability that makes possible its multiple biological roles. These data have established the concept that the Ras signal, instead of being one single, homogeneous entity, results from the integration of multiple, site-specified subsignals, and Ras has become a paradigm of how space can differentially shape signaling.

Keywords: GTPases; Ras; acylation; signal compartmentalization.

Figure 1.

Ras trafficking routes among subcellular compartments. Ras proteins, synthesized in cytoplasmic polysomes, are farnesylated and bind to the endoplasmic reticulum (ER), where they are processed (dark blue arrows). Therein, H-Ras and N-Ras are palmitoylated and subsequently translocated to the Golgi complex (GC) and to recycling endosomes from where they are transported, via vesicular transport, to their final destination in plasma membrane (PM) lipid rafts and disordered membrane microdomains (light blue arrows). K-Ras4B is transported by an unknown mechanism, probably mediated by microtubules to disordered membrane microdomains at the PM (brown arrow). At the PM, H-Ras can diffuse between lipid rafts and disordered membrane microdomains depending on its activation state (red arrow). At the PM, H-Ras and N-Ras are depalmitoylated and sent back to the GC, closing the acylation cycle (light blue arrows). The trafficking mechanism is unclear and could involve RhoGDI-like chaperones. H-Ras and N-Ras can also be internalized through the endocytic pathway via endosomes (dashed orange arrow). At the PM, K-Ras4B can be phosphorylated by protein kinase C (PKC), dislodging it from the PM to endomembranes, particularly mitochondria (green arrow). Also at the PM, H-Ras and N-Ras can be ubiquitinated, promoting their association to endosomes (purple arrow). Calmodulin binding to K-Ras4B at the PM brings about its recruitment to endosomes and the GC (orange arrow).