Posttranslational Modifications of RAS Proteins

Abstract

The three human RAS genes encode four proteins that play central roles in oncogenesis by acting as binary molecular switches that regulate signaling pathways for growth and differentiation. Each is subject to a set of posttranslational modifications (PTMs) that modify their activity or are required for membrane targeting. The enzymes that catalyze the various PTMs are potential targets for anti-RAS drug discovery. The PTMs of RAS proteins are the focus of this review.

Figure 1.

Differential posttranslational processing regulates RAS isoform subcellular trafficking. The trafficking of RAS isoforms HRAS, NRAS, KRAS4A, and KRAS4B (color coded, see legend) are schematized for the nascent proteins following messenger RNA (mRNA) translation on polysomes in the cytosol. The enzymes that catalyze the posttranslational modifications (PTMs) that direct trafficking are highlighted in dialog boxes (FTase, farnesyltransferase; RCE1, Ras-converting enzyme 1 protease; ICMT, isoprenylcysteine carboxylmethyltransferase; PATs, DHHC family of palmitoyl acyl transferases) with irreversible modifications (farnesylation and proteolysis) indicated by solid arrows, and reversible modifications (carboxyl methylation, palmitoylation, and phosphorylation) indicated by outlined arrows (see legend). Lipid modifications (farnesylation and palmitoylation) are color coded (see legend). The subcellular locations at which these modifications occur (ER, endoplasmic reticulum; Golgi, Golgi apparatus; RE, recycling endosome; cytosol, plasma membrane) are also indicated. Between the indicated subcellular compartments, the color-coded arrows denote both the RAS isoform undergoing transit and the direction of trafficking. For KRAS4A and KRAS4B, (+) signs indicate polybasic residues that confer electrostatic affinity for phospholipid headgroups, which, in the case of KRAS4B, is interrupted by phosphorylation (via protein kinase C [PKC] and GMP-dependent protein kinase 2 [PKG2] as discussed in the text but not depicted here). For NRAS, arrows indicate transit to the cytosol, a compartment in which this isoform accumulates when unpalmitoylated, from both the ER and plasma membrane. Prenyl-dependent chaperone interactions for N/KRAS with PDE6δ or for NRAS with VPS35 are not depicted but are discussed in the text.

Figure 2.

Posttranslational modifications (PTMs) of RAS isoforms. The full length of RAS is depicted schematically and includes partitioning of the G domain (aa1–165) and the hypervariable region (HVR) (aa166–188/189). Color coding of the HVR distinguishes each RAS isoform (HRAS, NRAS, KRAS4A, and KRAS4B). PTMs occurring within the G domain are indicated (glucosylation, sumolyation, ADP-ribosylation, acetylation, ubiquitination, and nitrosylation). For the HVR, unique color coding highlights residues modified by the various PTMs (phosphorylation, cis-trans [prolyl] isomerization, palmitoylation, farnesylation, proteolysis, and carboxyl methylation). Polybasic residues in the HVRs of KRAS4A and KRAS4B are indicated by black text. HVR modifications are correlated to the RAS isoforms on which they occur via isoform color-coded linkers, and also via the isoform color-coded text for the modified amino acids listed within the modifications dialog boxes.