Question

Identify the point in mitosis at which separase cleaves the protein complex that holds sister chromatid pairs together. In normal cells, separase is kept in an inactive state until it is needed.

Explain how the progression of cells past sequential cell cycle checkpoints and the activity of enzymes such as separase is controlled by interactions between two major groups of regulatory
proteins

Answer 1

1. The point of cleavage of the cohesin complex differes in vertebrates and invertebrates. In vertebrates, the cohesin complex is cleaved by separase enzymes at the onset of anaphase.

2. Cell cycle checkpoints and the cleavage of the cohesin complex are controlled by regulatory proteins called cyclins, cyclin dependent protein kinases (CdKs) and the Anaphase Promoting Complex/ Cyclosome (APC/C).

Step-by-step explanation:

(1)

Chromatid Cohesion:

Before the onset of anaphase in the cell cycle as well as in normal cells, a stable cohesion of the sister chromatids of chromosomes are crucial for normal cellular function and cell cycle progression. In vertebrates, sister chromatids are held together by the cohesin complex which is a complex of four sub-units: an α-kleisin subunit (SCC1/MCD1/RAD21), SCC3 (known as SA1 or SA2 in vertebrates), SMC1, and SMC3.

Cohesin Cleavage

In vertebrates, the cleavage of this cohesin complex occurs in two steps during the cell cycle. During prophase, the protein SA1/2 is phosphorylated. This detaches most of the cohesin complex from the chromosome apart from the centromeres.

Upon the onset of anaphase, the separase enzyme cleaves the SCC1 cohesin, detaching the two sister chromatids from each other.

Separase in Normal Cells:

In normal cells and in cells undergoing other phases of mitosis, separase is inhibited and even kept out of the nuclues to avoid contact with the cohesin complex on chromosomes. Separase inhibition occurs through two ways:

1. Binding and inactivation by a protein called securin.
2. Phosphorylation of serine 1126 (S1126) of separase.

(2)

Regulatory Proteins in Cell Cycle Checkpoints:

Cell cycle progression and cohesin cleavage by separase is regulated by two groups of regulatory proteins: cyclins, cyclin dependent protein kinases (CdKs) and the Anaphase Promoting Complex/ Cyclosome (APC/C).

These cell cycle checkpoints are crucial to ensure normal cell functioning and the successful error-free completion of cell division.

Cyclins and Cyclin Dependent Protein Kinases (CdKs):

Cyclins are proteins that are core regulators of the cell cycle. Four types of cyclins are found in eukaryotes: G1 cyclin, G1/S cyclin, S cyclin and M cyclin.

Cyclins acts as activators for enzymes called cyclin dependent protein kinases (CdKs). When a cyclin binds to the CdK, the CdK is activated and it phosphorylates target protein required for the initiation of the next phase of the cell cycle. CdKs are negatively regulated by the phosphorylation of specific sites other than the ones that initially activated the CdK.

Anaphase Promoting Complex/ Cyclosome (APC/C):

The APC/C causes the cell to move out of the M phase and into the G1 phase. This regulatory complex also influences the cleavage of the cohesion complex of the sister chromatids.

APC/C works by placing a tag protein, ubiquitin, on enzymes and regulatory proteins involved in cohesin cleavage. When APC/C tags the cohesin complex inhibitor, securin, the cleavage enzyme separase is activated. Separase then cleaves the cohesin complex of the sister chromatids, causing them to be separated.