Researchers at the Division of Structural Biology (STRUBI) at the Centre have found an explanation for the distinctive behaviour of a group of receptors crucial for cell to cell communication. Elena Seiradake, Radu Aricescu, Yvonne Jones, and colleagues,
recently published their finding in Nature Structural and Molecular Biology (1). Their work will contribute to a better understanding of how the receptors control key processes during development and repair of the nervous system, as well as in many cancers.
The receptors, known as Eph (Erythropoetin-producing hepatoma) receptors, sit in the cell membrane. They show an intriguing, but until now, unexplained behaviour when activated which is quite different from the behaviour of other receptors. The process is triggered by proteins called ephrins which bind to the external portion of the receptor. This causes the receptors to cluster together at the cell surface enabling a signal to be transmitted internally.
To understand the architecture of these receptor assemblies, the group used X-ray crystallography. They focused on human EphA2, an Eph receptor that is known to play a key role in cancer, and looked at the whole of the extracellular portion of the receptor (eEphA2).
The group solved the structure of eEphA2 alone and when bound to the receptor-binding portion of its activating protein, ephrinA5. They found that even in the absence of ephrin A5, eEphA2 molecules cluster together into arrays in the crystal. Binding of ephrinA5 snaps the receptor arrays into a slightly different configuration so that they form a more elaborate assembly.
To investigate whether these interactions between receptor molecules are biologically relevant, the researchers took advantage of the microscopy facilities and expertise available at the Centre.
Figure: Cells with EphA2 (multi-coloured) and ephrinA5 (light blue). The model suggests that a few ephrin molecules seed formation of large signalling clusters made up of receptor and ephrin coming from both cells. From: Nature Structural and Molecular Biology.
Microscopy was used to track EphA2 movement inside epithelial cells by tagging this protein with a fluorescent label. When labelled EphA2 is introduced into these cells, the receptor specifically accumulates at cell-cell contacts - a consequence of receptors on neighbouring cells interacting with each other and with ephrinA5 which the cells make naturally.
The crystallographic results indicated that specific regions of the receptor interact in the ephrin-triggered assembly. When the researchers tested the importance of these regions on the behaviour of the receptor using removal experiments, they found that they were critical for both clustering at cell-cell contacts and for receptor activation.
This indicates that the structure of the receptor assembly seen in the crystal is functionally important - shedding light on what is happening in the cell membrane when the receptor in activated.
The findings, says Professor Jones, suggest that receptor activation may occur by a 'nucleation' mechanism. Just a few ephrin-Eph receptor interactions can pull Eph receptors in from further afield, facilitating more ephrin binding and so extending the area of signalling. The question now is whether other cell-surface receptor systems may be working in a similar way.
Dr Seiradake will be pursuing these studies further with funding from a Marie Curie Fellowship.
(1) Reference PMID 20228801
Seiradake E, Harlos K, Sutton G, Aricescu AR, Jones EY. An extracellular steric seeding mechanism for Eph-ephrin signaling platform assembly. Nat Struct Mol Biol. 2010 Apr;17(4):398-402
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