From Gene to Structure: Exploring MC4R in Unliganded and Ligand-Bound
Sample stability often decides whether a GPCR cryo-EM structure succeeds, yet standard purification metrics can miss early signs of complex instability. For the melanocortin-4 receptor (MC4R), a Class A GPCR central to appetite regulation, the agonist setmelanotide made the difference between a homogeneous complex and one that partially dissociated.
A Gene-to-Structure Case Study of MC4R
This white paper follows the MC4R-Gs-scFv16 complex from insect cell expression to cryo-EM structure, prepared with and without setmelanotide. Both preparations looked comparable by SDS-PAGE and standard purification metrics, but analytical SEC and negative-stain EM showed that the unliganded complex was less stable. Both were still carried through to cryo-EM, giving structures at 2.6 Å (setmelanotide-bound; PDB ID 40BG) and 3.1 Å (unliganded; PDB ID 40BF), with only 55,609 of 445,504 picked particles contributing to the unliganded map. SPR on the purified complex gave a mean KD of 0.21 ± 0.09 nM for setmelanotide.
What You’ll Learn in the White Paper
- How analytical SEC and negative-stain EM identify unstable GPCR complexes before high-resolution data collection
- How the absence of agonist affects particle yield and map quality in cryo-EM
- What the 2.6 Å structure shows about setmelanotide recognition, including the calcium-mediated bridge between receptor and peptide
- How SPR was used to measure agonist affinity for a purified signaling complex, and why KD values should not be directly compared with Ki or EC₅₀ values from other assay formats
Download the white paper for the full methods, cryo-EM processing details, SPR data, and practical guidance on integrating sample quality control with structure determination.
Citations & References
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