Evolutionary divergence in the catalytic activity of the CAM-1, ROR1 and ROR2 kinase domains

Area of Science:

• Molecular Biology
• Cell Signaling
• Evolutionary Biology

Background:

• Receptor tyrosine kinase-like orphan receptors (ROR) 1 and 2 are atypical receptor tyrosine kinases (RTKs) implicated in human diseases.
• The kinase function of ROR1 and ROR2 is controversial due to deviations in their ATP-binding domains.
• ROR2 has been shown to suppress canonical Wnt/β-catenin signaling via a Wnt-responsive, β-catenin-independent pathway.

Purpose of the Study:

• To investigate the catalytic activity and signaling mechanisms of ROR1 and ROR2.
• To explore the evolutionary divergence of ROR signaling pathways.
• To determine the role of ROR extracellular domains in Wnt pathway regulation.

Main Methods:

• Biochemical assays to assess the tyrosine kinase activity of ROR1, ROR2, and their homologs.
• Site-directed mutagenesis to substitute consensus and non-consensus residues.
• Wnt signaling assays using Wnt3a and Wnt5a to evaluate the function of ROR domains.

Main Results:

• ROR1 and ROR2 kinase domains are catalytically deficient, unlike the active CAM-1 homolog in C. elegans.
• Substitution of non-consensus residues into CAM-1 significantly reduced kinase activity.
• The extracellular domains of ROR1 and ROR2 alone suppressed canonical Wnt3a signaling and enhanced Wnt5a-mediated suppression.

Conclusions:

• Human ROR1 and ROR2 function as pseudokinases, lacking significant catalytic activity.
• The divergence in kinase activity between vertebrate RORs and C. elegans CAM-1 highlights evolutionary changes in ROR signaling.
• The extracellular domains of ROR1 and ROR2 play a crucial role in regulating Wnt signaling pathways.