The Ras-Byr2RBD complex: Structural basis for Ras effector recognition in yeast

Background-The small GTP binding protein Ras has important roles in cellula r growth and differentiation. Mutant Ras is permanently active and contribu tes to cancer development. In its activated form, Ras interacts with effect or proteins, frequently initiating a kinase cascade. In the lower eukaryoti c Schizosaccharomyces pombe, Byr2 kinase represents a Ras target that In te rms of signal-transduction hierarchy can be considered a homolog of mammali an Raf-kinase. The activation mechanism of protein kinases by Ras is not un derstood, and there is no detailed structural information about Ras binding domains (RBDs) in nonmammalian organisms. Results: The crystal structure of the Ras-Byr2RBD complex at 3 Angstrom res olution shows a complex architecture similar to that observed in mammalian homologous systems, with an interprotein beta sheet stabilized by predomina ntly polar interactions between the interacting components. The C-terminal half of the Ras switch I region contains most of the contact anchors, while on the Byr2 side, a number of residues from topologically distinct regions are Involved in complex stabilization. A C-terminal helical segment, which is not present in the known mammalian homologous systems and which is part of the auto-inhibitory region, has an additional binding site outside the switch I region. Conclusions: The structure of the Ras-Byr2 complex confirms the Ras binding module as a communication element mediating Ras-effector interactions; the Ras-Byr2 complex is also conserved in a lower eukaryotic system like yeast , which is in contrast to other small GTPase families. The extra helical se gment might be involved in kinase activation.