Building bodies is a complicated business. Thousands of growing cell types, which all need to end up in the right place, doing the right job. Even when we are fully grown, our cells and tissues still need to communicate, and new cells grow and migrate to repair and renew our tissues. To assist with this huge task, the body uses signalling molecules and receptors to direct and guide cells, and to allow communication between different cells in the body. These signals act like traffic lights, stopping or allowing growth and cell movement.
Repulsive Guidance Molecules (RGMs) are a family of signalling molecules with multiple functions in human biology. There are three key family members, responsible for guiding axons in the developing nervous system, for which they were named, iron metabolism and immune cell regulation, amongst others. Mutations in the genes which code for the RGMs can lead to diseases including juvenile hemochromatosis, an iron storage disorder, and the molecules have been implicated in the development of both multiple sclerosis and cancer.
Repulsive Guidance Molecule (RGM) acting as a molecular staple for its receptor Neogenin (NEO1). (A) The crystal structure of the RGM-NEO1 complex suggests a mechanism for signal transduction through the plasma membrane. (B) Axon growth assays confirm the observed higher order signalling complex.
Until now, the means by which the RGM signal connected to its receptor was unknown. Researchers from the Division of Structural Biology, Oxford, have now used Diamond Light Source to solve the structure of the Neogenin receptor in complex with the RGMB signalling molecule (RGMB is one of the family members – also known as Dragon), giving new insight into the mechanism of the signal-receptor pair. The group observed that the RGMB acts as a ‘molecular staple’, bringing together two receptors at a major and minor binding site. This occurs in a 2:2 oligomeric arrangement.
The research also revealed a novel protein fold in RGMB, which is strongly associated with the function of the protein. Mutations in the region of the fold are already known to be associated with the development of juvenile hemochromatosis.
Using site directed mutagenesis, the group established the relevance of the 2:2 oligomeric arrangement to the function of the signal, by investigating variants with mutations at both the major and minor binding site. Observing the growth of axons in both mutants confirmed that the arrangement is vital for function of the RGM signal. The group also investigated the role of pH in the function of the signal-receptor complex, finding that the complex dissociates with increasing acidity, consistent with the conditions found where the complex is internalised within the cell.
Christian Siebold, from the Division of Structural Biology, Oxford University, who led the work says “Diamond was crucial to revealing the molecular staple mechanism at work in this signal-receptor complex. We were able to observe the complex in multiple states using the I03 beamline, an amazing achievement which was really key to solving this puzzle.”