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Published on: October 7, 2021
Dissection of c-MOS degron
Jun Sheng1, Akiko Kumagai, William G Dunphy
1Division of Biology, 147-75, California Institute of Technology, 1200 East California Blvd, Pasadena, CA 91125, USA.
Abstract:
c-MOS, a MAP kinase kinase kinase, is a regulator of oocyte maturation. The concentration of c-MOS is controlled in part through its conditional degradation. Previous studies proposed the "second-codon rule", according to which the N-terminal proline (Pro) of c-MOS is a destabilizing residue that targets c-MOS for degradation. We analyzed the degradation signal (degron) of c-MOS in Xenopus oocytes, found it to be a portable degron, and demonstrated that, contrary to the model above, the N-terminal Pro residue of c-MOS is entirely dispensable for its degradation if Ser-2 (encoded Ser-3) of c-MOS is replaced by a small non-phosphorylatable residue such as Gly. The dependence of c-MOS degradation on N-terminal Pro is shown to be caused by a Pro-mediated downregulation of the net phosphorylation of Ser-2, a modification that halts c-MOS degradation in oocytes. Thus, the N-terminal Pro residue of c-MOS is not a recognition determinant for a ubiquitin ligase, in agreement with earlier evidence that Pro is a stabilizing residue in the N-end rule.
Insights
The N-terminal Pro residue of c-MOS is not essential for its degradation in oocytes. Degradation depends on Ser-2 phosphorylation, not Pro, challenging previous models of c-MOS regulation.
Area of Science:
- Cell Biology
- Molecular Biology
- Developmental Biology
Background:
- c-MOS (cytoplasmic-only MOS) is a MAP kinase kinase kinase crucial for oocyte maturation.
- Protein concentration is regulated by conditional degradation.
- The N-terminal proline (Pro) of c-MOS was previously thought to target it for degradation via the 'second-codon rule'.
Purpose of the Study:
- To investigate the degradation signal (degron) of c-MOS in Xenopus oocytes.
- To determine the role of the N-terminal Pro residue in c-MOS degradation.
- To elucidate the mechanism regulating c-MOS protein stability.
Main Methods:
- Analysis of c-MOS degradation signals in Xenopus oocytes.
- Site-directed mutagenesis to alter the N-terminal Pro residue and Ser-2 phosphorylation status.
- Assessment of c-MOS protein stability and phosphorylation levels.
Main Results:
- The N-terminal Pro residue of c-MOS is dispensable for its degradation.
- Replacing Ser-2 with Gly renders the N-terminal Pro irrelevant for degradation.
- N-terminal Pro downregulates Ser-2 phosphorylation, which normally halts c-MOS degradation.
Conclusions:
- The N-terminal Pro residue of c-MOS does not function as a degron recognized by ubiquitin ligases.
- c-MOS degradation is primarily regulated by the phosphorylation status of Ser-2.
- This finding refutes the 'second-codon rule' model for c-MOS degradation and highlights a novel regulatory mechanism involving proline and phosphorylation.
