A secretory kinase complex regulates extracellular protein phosphorylation

Area of Science:

• Biochemistry
• Cell Biology
• Molecular Biology

Background:

• Extracellular phosphoproteins are known, but protein kinases in the secretory pathway remain largely uncharacterized.
• Fam20C is a Golgi casein kinase crucial for biomineralization by phosphorylating secreted proteins.
• Fam20A, a paralog of Fam20C, is vital for enamel formation, yet its biochemical role is undefined.

Purpose of the Study:

• To elucidate the biochemical function of Fam20A.
• To investigate the interaction between Fam20A and Fam20C.
• To understand the regulation of extracellular protein phosphorylation in the secretory pathway, particularly in enamel formation.

Main Methods:

• In vitro kinase assays to assess Fam20A's effect on Fam20C activity.
• Cell-based experiments to study protein phosphorylation in enamel matrix proteins.
• Biochemical analysis to characterize the functional complex formed by Fam20A and Fam20C.

Main Results:

• Fam20A was identified as a potentiator of Fam20C kinase activity.
• Fam20A promotes the phosphorylation of enamel matrix proteins both in vitro and in cellular models.
• Fam20A functions as a pseudokinase that forms a complex with Fam20C, enhancing extracellular phosphorylation.

Conclusions:

• Fam20A and Fam20C collaborate to regulate extracellular protein phosphorylation, critical for enamel biomineralization.
• This study provides the first insights into the regulatory mechanisms governing phosphorylation within the secretory pathway.
• The findings uncover a novel pseudokinase-kinase interaction essential for a key biomineralization process.