Phosphorylation of the spindle checkpoint protein Mad2 regulates its conformational transition

Area of Science:

• Cellular biology
• Molecular mechanisms of signal transduction
• Protein conformational dynamics

Background:

• Regulated protein conformational changes are vital for cellular signal transduction.
• Mad2, a spindle checkpoint protein, exists in two forms: open (O-Mad2) and closed (C-Mad2).
• C-Mad2 inhibits Cdc20, an APC/C activator, to prevent premature anaphase during mitosis.

Purpose of the Study:

• To investigate the role of Mad2 phosphorylation in regulating its conformational changes and function.
• To elucidate how phosphorylation at S195 affects Mad2's interactions with binding partners.
• To understand the impact of altered Mad2 function on cellular processes like the spindle checkpoint.

Main Methods:

• Site-directed mutagenesis to create phospho-mimicking Mad2(S195D) mutant.
• Intein-mediated protein semisynthesis to obtain phospho-S195 Mad2.
• Analysis of Mad2 binding affinities to ligands (Mad1, MBP1, Cdc20).
• Functional assessment of Mad2(S195D) overexpression in human cells.

Main Results:

• Phosphorylation at S195 does not induce C-Mad2 formation independently.
• Mad2(S195D) exhibits impaired binding to Cdc20 but retains binding to Mad1 and MBP1.
• Overexpression of Mad2(S195D) leads to spindle checkpoint defects in human cells.
• Mad2 phosphorylation differentially regulates binding to Mad1 and Cdc20.

Conclusions:

• Mad2 phosphorylation at S195 inhibits its function by modulating ligand interactions.
• The conformational change of Mad2 is subject to posttranslational regulation.
• Understanding Mad2 phosphorylation provides insights into spindle checkpoint control and cell cycle regulation.