Phosphorylation of the spindle checkpoint protein Mad2 regulates its conformational transition

Regulated conformational changes of proteins are critical for cellular signal transduction. The spindle checkpoint protein Mad2 is an unusual protein with two native folds: the latent open conformer (O-Mad2) and the activated closed conformer (C-Mad2). During mitosis, cytosolic O-Mad2 binds to the Mad1-Mad2 core complex at unattached kinetochores and undergoes conformational activation to become C-Mad2. C-Mad2 binds to and inhibits Cdc20, an activator of APC/C, to prevent precocious anaphase onset. Here, we show that the conformational transition of Mad2 is regulated by phos- phorylation of S195 in its C-terminal region. The phospho-mimicking Mad2^S195D mutant and the phospho-S195 Mad2 protein obtained using intein-mediated semisynthesis do not form C-Mad2 on their own. Mad2^S195D fails to bind to Cdc20, a low-affinity ligand, but still binds to high-affinity ligands, such as Mad1 and MBP1, forming ligand-bound C-Mad2. Overexpression of Mad2^S195D in human cells causes checkpoint defects. Our results indicate that Mad2 phosphor- ylation inhibits its function through differentially regulating its binding to Mad1 and Cdc20 and establish that the conformational change of Mad2 is regulated by posttranslational mechanisms.