Final answer:
The inhibitory protein in the spindle assembly checkpoint of cell division that leads to the activation of Separase and subsequent degradation of cohesins is Securin.
Step-by-step explanation:
In the spindle assembly checkpoint of cell division, Securin is an inhibitory protein that causes the activation of Separase which degrades cohesins to allow chromosome disjunction and segregation. The M checkpoint, or spindle checkpoint, ensures that all the sister chromatids are correctly attached to the spindle microtubules. It prevents the cell from proceeding to anaphase until the tension in the spindle apparatus pulls at the kinetochores of each pair of sister chromatids, confirming that they are firmly anchored to at least two spindle fibers arising from opposite poles of the cell. Passing the M checkpoint indicates that the cell will complete mitosis and cytokinesis, leading to the formation of two daughter cells, each entering a new G1 phase.