Митоздық тежелуге жауапты Mad2 протеині және оның ролі

Mad2 (mitotic arrest deficient 2) is an essential spindle checkpoint protein. The spindle checkpoint system is a regulatory system that restrains progression through the metaphase to anaphase transition. The Mad2 gene was first identified in the yeast S. cerevisiae in a screen for genes which when mutated would confer sensitivity to microtubule poisons. The human orthologues of Mad2 (MAD2L1 and MAD2L2) were first cloned in a search for human cDNAs that would rescue the microtubule poison sensitivity of a yeast strain in which a kinetochore binding protein was missing. The protein was shown to be present at unattached kinetochores and antibody inhibition studies demonstrated it was essential to execute a block in the metaphase to anaphase transition in response to the microtubule poison nocodazole.

Progression from metaphase to anaphase is marked by sister chromatid separation. The cell cycle surveillance mechanism that prevents sister chromatid separation and transition into anaphase is called the spindle checkpoint. As a safeguard against chromosome segregation errors, the spindle assembly checkpoint (SAC) delays anaphase until all sister chromatid pairs have become bipolarly attached. Once microtubules attach to kinetochores, chromosomes are aligned on the metaphase plate, and proper bi orientation has been achieved, the SAC stopping mechanisms are removed. Entrance into anaphase is mediated by APCCdc20 activation. APCCdc20 is a ubiquitin protein ligase that tags the protein, securin, for destruction. Securin destruction liberates and activates its bound protease partner, separase. Separase bound to securin remains inhibited; however, when inhibition is relieved, activated separase cleaves the cohesin complex which links the sister chromatids together. Without Cdc20, the anaphase promoting complex (APC) cannot become activated and anaphase is not triggered. Mad2 was shown to inhibit the activity of the APC by direct physical interaction in a ternary complex with Cdc20. Kinetochores that remain unattached to microtubules catalyze the sequestration of Cdc20 by Mad2. In fact, when metaphase mammalian cells are treated with the spindle depolymerizing agent nocodazole, Mad2 proteins become localized at the kinetochores of all sister chromatid pairs. It is speculated that once formed, Cdc20:Mad2 complexes can amplify the anaphase wait signal by stimulating further conversion of cytosolic Open Mad2 and free Cdc20 into more Cdc20:Closed Mad2 complexes. This diffusible signal propagation away from the kinetochore complexes could account for how vacancy of just one tiny kinetochore site can completely shut down the metaphase to anaphase transition.

Much remains to be explained about spindle checkpoint signaling and the contribution of other spindle checkpoint assembly proteins such as Bub1, BubR1, and Bub3. BubR1 and Bub3 can also form complexes with Cdc20, but it remains to be seen if these proteins facilitate Cdc20 binding to Open Mad2.