Securin and Separase Phosphorylation Act Redundantly to Maintain Sister Chromatid Cohesion in Mammalian Cells

Abstract

The spindle assembly checkpoint monitors the integrity of the spindle microtubules, which attach to sister chromatids at kinetochores and play a vital role in preserving genome stability by preventing missegregation. A key target of the spindle assembly checkpoint is securin, the separase inhibitor. In budding yeast, loss of securin results in precocious sister chromatid separation when the microtubule spindle is disrupted. However, in contrast to budding yeast, mammalian securin is not required for spindle checkpoint, suggesting that there are redundant mechanisms controlling the dissolution of sister chromatid cohesion in the absence of securin. One candidate mechanism is the inhibitory phosphorylation of separase. We generated a nonphosphorylable point mutant (S1121A) separase allele in securin Ϫ/Ϫ mouse embryonic stem cells. Securin Ϫ/Ϫ separase ϩ/S1121A cells are viable but fail to maintain sister chromatid cohesion in response to the disruption of spindle microtubules, show enhanced sensitivity to nocodazole, and cannot recover from prometaphase arrest.

Pttg1/securin is required for the branching morphogenesis of the mammary gland and suppresses mammary tumorigenesis

“…We have shown previously in cells and in mice that securin and separase phosphorylation were redundant functionally (17). It is also very unlikely that mitotic errors (if any) caused by the loss of Pttg1 could account for the profound changes in gene expression.…”

“…In vertebrates, there are two mechanisms to inhibit separase, phosphorylation and binding by securin (16). We have shown previously that these two mechanisms are redundant in most somatic cells (17), but, in mice, postmigration germ cells rely entirely on phosphorylation to inhibit separase (18). Mammalian securin, PTTG1, has been associated with cancer before it is known as securin.…”

“…Most likely, this is because in vertebrates separase is also inhibited by phosphorylation (16). Only when both inhibitory mechanisms are removed are defects in mitotic separation of sister chromatids seen (17,18). Nonetheless, the function of securin as the inhibitor of separase fueled the speculation that the overexpression of PTTG1 in tumors might be related to chromosome instability.…”

Anaphase Topsy-Turvy: Cdk1 a Securin, Separase a CKI

“…Surprisingly, while knock-down experiments indicate that separase is essential, securin knock-out mice are largely normal and corresponding ES cells do not exhibit gains or losses of chromosomes relative to wild type ES cells. 8,9 The mild phenotype in the mouse model contrasts with the high rate of chromosome missegregation reported for a human colon cancer cell line upon deletion of both copies of securin. 10 Pfleghaar et al have recently shed light on this contradiction by showing that transient chromosomal instability in the human securin -/-cell line soon gave way to chromosomal stability.…”

DNA-dependent cohesin cleavage by separase

“…22 Interestingly, however, mouse embryonic stem cells lacking both these separase regulations can still progress through mitosis in a timely fashion with correct chromosome segregation. 23 The mutant cells separate chromosomes prematurely only when challenged with a microtubule poison. Therefore the pathways that temporally regulate separase are not yet fully elucidated in vertebrate cells.…”