The Ras-Membrane Interface: Isoform-Specific Differences in the Catalytic Domain

摘要

Abstract The small GTPase Ras is mutated in about 20% of human cancers, primarily at active site amino acid residues G12, G13, and Q61. Thus, structural biology research has focused on the active site, impairment of GTP hydrolysis by oncogenic mutants, and characterization of protein-protein interactions in the effector lobe half of the protein. The C-terminal hypervariable region has increasingly gained attention due to its importance in H-Ras, N-Ras, and K-Ras differences in membrane association. A high-resolution molecular view of the Ras-membrane interaction involving the allosteric lobe of the catalytic domain has lagged behind, although evidence suggests that it contributes to isoform specificity. The allosteric lobe has recently gained interest for harboring potential sites for more selective targeting of this elusive "undruggable" protein. The present review reveals critical insight that isoform-specific differences appear prominently at these potentially targetable sites and integrates these differences with knowledge of Ras plasma membrane localization, with the intent to better understand the structure-function relationships needed to design isoform-specific Ras inhibitors. The Allosteric Lobe Is a Site of Ras-Membrane Interactions The small GTPase Ras functions as a molecular switch (1), at the center of which is the exchange of GDP to GTP by GEFs (guanine nudeotide exchange factors) and hydrolysis of GTP to GDP, activated by GAPs (GTPase-activating proteins; ref. 2). Ras proteins are lipidated at their C-terminal hypervariable region (HVR), which inserts into the membrane, whereas the catalytic G domain functions at the cytoplasmic interface with the membrane (3). Here, many effector and regulator proteins interact with the effector lobe (residues 1-86) of GTP-bound Ras to promote signaling cascades that translate to changes in gene expression, controlling cellular outcomes, such as cell proliferation, differentiation, and apoptosis (4). Point mutations occur most frequently at residue positions 12, 13, and 61, leading to constitutive Ras activation and hyperproliferation traditionally associated with cancer and developmental disorders (5).