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Role of the histone domain in the autoinhibition and activation of the Ras activator Son of Sevenless.

ABSTRACT: Membrane-bound Ras is activated by translocation of the Son of Sevenless (SOS) protein to the plasma membrane. SOS is inactive unless Ras is bound to an allosteric site on SOS, and the Dbl homology (DH) and Pleckstrin homology (PH) domains of SOS (the DH-PH unit) block allosteric Ras binding. We showed previously that the activity of SOS at the membrane increases with the density of PIP(2) and the local concentration of Ras-GTP, which synergize to release the DH-PH unit. Here we present a new crystal structure of SOS that contains the N-terminal histone domain in addition to the DH-PH unit and the catalytic unit (SOS(HDFC), residues 1-1049). The structure reveals that the histone domain plays a dual role in occluding the allosteric site and in stabilizing the autoinhibitory conformation of the DH-PH unit. Additional insight is provided by kinetic analysis of the activation of membrane-bound Ras by mutant forms of SOS that contain mutations in the histone and the PH domains (E108K, C441Y, and E433K) that are associated with Noonan syndrome, a disease caused by hyperactive Ras signaling. Our results indicate that the histone domain and the DH-PH unit are conformationally coupled, and that the simultaneous engagement of the membrane by a PH domain PIP(2)-binding interaction and electrostatic interactions between a conserved positively charged patch on the histone domain and the negatively charged membrane coincides with a productive reorientation of SOS at the membrane and increased accessibility of both Ras binding sites on SOS.

SUBMITTER: Gureasko J

PROVIDER: S-EPMC2816639 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Computational docking and solution x-ray scattering predict a membrane-interacting role for the histone domain of the Ras activator son of sevenless.

Project description:The Ras-specific nucleotide exchange factor son of sevenless (SOS) is a large, multidomain protein with complex regulation, including a Ras-dependent allosteric mechanism. The N-terminal segment of SOS, the histone domain, contains two histone folds, which is highly unusual for a cytoplasmic protein. Using a combination of computational docking, small-angle x-ray scattering, mutagenesis, and calorimetry, we show that the histone domain folds into the rest of SOS and docks onto a helical linker that connects the pleckstrin-homology (PH) and Dbl-homology (DH) domains of SOS to the catalytic domain. In this model, a positively charged surface region on the histone domain is positioned so as to provide a fourth potential anchorage site on the membrane for SOS in addition to the PH domain, the allosteric Ras molecule, and the C-terminal adapter-binding site. The histone domain in SOS interacts with the helical linker, using a region of the surface that in nucleosomes is involved in histone tetramerization. Adjacent surface elements on the histone domain that correspond to the DNA-binding surface of nucleosomes form the predicted interaction site with the membrane. The orientation and position of the histone domain in the SOS model implicates it as a potential mediator of membrane-dependent activation signals.

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