RET Functions as a Dual-Specificity Kinase that Requires Allosteric Inputs from Juxtamembrane Elements

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Signaling

Background:

• Receptor tyrosine kinases (RTKs) display diverse activation mechanisms despite conserved catalytic domains.
• Activation relies on extracellular ligand-binding domains, variable intracellular sequences, and autophosphorylation patterns.

Purpose of the Study:

• To elucidate the role of the juxtamembrane (JM) segment in RET kinase activation.
• To investigate the contribution of specific phosphorylation sites, including Y687 and S909, to RET function.
• To uncover the allosteric mechanisms governing RET signaling.

Main Methods:

• Structure-function analyses of RET kinase domain and juxtamembrane segment.
• Site-directed mutagenesis to investigate phosphorylation site requirements.
• Biochemical assays to assess kinase activity and conformational changes.
• In vivo studies in Drosophila to confirm functional relevance.

Main Results:

• The JM segment enhances RET catalytic activity through phosphorylation at Y687.
• Y687 phosphorylation is essential for the JM region to rescue catalytically deficient RET mutants.
• A novel allosteric mechanism involving JM hinge, αC helix, and activation-loop serine phosphorylation (S909) was identified.
• Phospho-S909 engages the HRD motif, promoting phospho-tyrosine accessibility and regulatory spine assembly.
• RET possesses intrinsic dual-specificity kinase activity, responsible for S909 phosphorylation.
• Phospho-S909 is critical for RET signaling in Drosophila.

Conclusions:

• RET activation involves dual-specificity kinase activity and allosteric regulation by the JM segment.
• The interplay between Y687 and S909 phosphorylation dictates RET kinase activity and signaling output.
• Understanding these mechanisms provides insights for targeted drug discovery in RET-dependent diseases.