A two-site flexible clamp mechanism for RET-GDNF-GFRα1 assembly reveals both conformational adaptation and strict geometric spacing.

RET receptor tyrosine kinase plays vital developmental and neuroprotective roles in metazoans. GDNF family ligands (GFLs) when bound to cognate GFRα co-receptors recognize and activate RET stimulating its cytoplasmic kinase function. The principles for RET ligand-co-receptor recognition are incompletely understood. Here, we report a crystal structure of the cadherin-like module (CLD1-4) from zebrafish RET revealing interdomain flexibility between CLD2 and CLD3. Comparison with a cryo-electron microscopy structure of a ligand-engaged zebrafish RETECD-GDNF-GFRα1a complex indicates conformational changes within a clade-specific CLD3 loop adjacent to the co-receptor. Our observations indicate that RET is a molecular clamp with a flexible calcium-dependent arm that adapts to different GFRα co-receptors, while its rigid arm recognizes a GFL dimer to align both membrane-proximal cysteine-rich domains. We also visualize linear arrays of RETECD-GDNF-GFRα1a suggesting that a conserved contact stabilizes higher-order species. Our study reveals that ligand-co-receptor recognition by RET involves both receptor plasticity and strict spacing of receptor dimers by GFL ligands.

An Integrated Chemical Proteomics Approach for Quantitative Profiling of Intracellular ADP-Ribosylation.

ADP-ribosylation is integral to a diverse range of cellular processes such as DNA repair, chromatin regulation and RNA processing. However, proteome-wide investigation of its cellular functions has been limited due to numerous technical challenges including the complexity of the poly(ADP-ribose) (PAR) chains, low abundance of the modification and lack of sensitive enrichment methods. We herein show that an adenosine analogue with a terminal alkyne functionality at position 2 of the adenine (2-alkyne adenosine or 2YnAd) is suitable for selective enrichment, fluorescence detection and mass spectrometry proteomics analysis of the candidate ADP-ribosylome in mammalian cells. Although similar labelling profiles were observed via fluorescence imaging for 2YnAd and 6YnAd, a previously reported clickable NAD+ precursor, quantitative mass spectrometry analysis of the two probes in MDA-MB-231 breast cancer cells revealed a significant increase in protein coverage of the 2YnAd probe. To facilitate global enrichment of ADP-ribosylated proteins, we developed a dual metabolic labelling approach that involves simultaneous treatment of live cells with both 2YnAd and 6YnAd. By combining this dual metabolic labelling strategy with highly sensitive tandem mass tag (TMT) isobaric mass spectrometry and hierarchical Bayesian analysis, we have quantified the responses of thousands of endogenous proteins to clinical PARP inhibitors Olaparib and Rucaparib.