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Published on: November 8, 2006
The Ras-Byr2RBD complex: structural basis for Ras effector recognition in yeast
K Scheffzek1, P Grünewald, S Wohlgemuth
1Max-Planck-Institut für molekulare Physiologie, Abt. Strukturelle Biologie, Otto-Hahn-Str. 11, 44227, Dortmund, Germany.
Background:
The small GTP binding protein Ras has important roles in cellular growth and differentiation. Mutant Ras is permanently active and contributes to cancer development. In its activated form, Ras interacts with effector proteins, frequently initiating a kinase cascade. In the lower eukaryotic Schizosaccharomyces pombe, Byr2 kinase represents a Ras target that in terms of signal-transduction hierarchy can be considered a homolog of mammalian Raf-kinase. The activation mechanism of protein kinases by Ras is not understood, 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 A resolution shows a complex architecture similar to that observed in mammalian homologous systems, with an interprotein beta sheet stabilized by predominantly 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 segment might be involved in kinase activation.
Insights
Researchers elucidated the Ras-Byr2 complex structure, revealing conserved Ras-effector interactions in yeast. An additional helical segment in Byr2 may play a role in kinase activation, offering insights into cancer-related signaling pathways.
Area of Science:
- Molecular Biology
- Structural Biology
- Biochemistry
Background:
- Ras GTPase is crucial for cell growth and differentiation, with mutations driving cancer by maintaining constitutive activity.
- Ras proteins activate downstream signaling cascades, often involving kinase pathways, but the precise activation mechanism remains unclear.
- Understanding Ras-effector interactions is vital, yet structural data for non-mammalian systems are limited.
Purpose of the Study:
- To determine the structural basis of Ras-effector interactions using the Schizosaccharomyces pombe Byr2 kinase as a model.
- To investigate the conserved mechanisms of Ras signaling in a lower eukaryotic system.
- To identify novel structural features that may contribute to kinase activation.
Main Methods:
- X-ray crystallography was employed to determine the structure of the Ras-Byr2 Ras binding domain (RBD) complex.
- The complex was analyzed at 3 Å resolution to elucidate interprotein interactions and structural architecture.
- Bioinformatic and structural comparisons were made with known mammalian Ras-effector complexes.
Main Results:
- The crystal structure revealed a conserved complex architecture between Ras and Byr2RBD, featuring an interprotein beta sheet stabilized by polar interactions.
- Key contact points were identified within the Ras switch I region and across topologically distinct regions of Byr2.
- A novel C-terminal helical segment in Byr2, absent in mammalian homologs, was found to possess an additional binding site outside the switch I region.
Conclusions:
- The Ras-Byr2 complex structure validates the Ras binding module as a conserved communication element mediating Ras-effector interactions.
- The conservation of Ras-effector complex architecture in yeast highlights a fundamental signaling mechanism across eukaryotes.
- The identified C-terminal helical segment in Byr2 is proposed to play a role in kinase activation, potentially representing a novel regulatory mechanism.
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