Sister chromatid separation during anaphase requires the proteolysis of cohesin by separase, whose activity depends on the degradation of securin. Securin, also known as human pituitary tumor-transforming gene 1 (PTTG1), is a multifunctional protein implicated in the control of mitosis and the pathogenesis of many tumor types. Securin is an anaphase inhibitor which prevents premature sister chromatid separation by blocking the activity of separase (ESP1).

Here I found that PTTG1 is the major securin isoform expressed in most normal and cancer cell lines. Unlike PTTG1, a highly homologous isoform PTTG2 did not interact with separase. This is consistent with the lack of effect of PTTG2 on mitosis even when it was expressed as a stabilized form. With the aim of mapping which part...[ Read more ]

Sister chromatid separation during anaphase requires the proteolysis of cohesin by separase, whose activity depends on the degradation of securin. Securin, also known as human pituitary tumor-transforming gene 1 (PTTG1), is a multifunctional protein implicated in the control of mitosis and the pathogenesis of many tumor types. Securin is an anaphase inhibitor which prevents premature sister chromatid separation by blocking the activity of separase (ESP1).

Here I found that PTTG1 is the major securin isoform expressed in most normal and cancer cell lines. Unlike PTTG1, a highly homologous isoform PTTG2 did not interact with separase. This is consistent with the lack of effect of PTTG2 on mitosis even when it was expressed as a stabilized form. With the aim of mapping which part(s) of PTTG1 contributes to the association with separase, a single amino acid (H¹³⁴) was identified to account for the loss of securin function in PTTG2 using chimeras between PTTG1 and PTTG2 and other experiments. Importantly, this motif is not conserved in PTTG2, explaining why PTTG2 failed to interact with separase. Mutation of that residue in PTTG1 significantly reduces the binding affinity between PTTG1 and separase. Conversely, PTTG2 gains association with separase after introduction of that residue (H¹³⁴) and becomes a functional securin which can prevent anaphase onset.

Evidence from different organisms indicates that securin not only inhibits separase, but also positively regulates separase. However, how securin promotes separase activity in organisms other than yeast remains incompletely understood. Here I found that PTTG1 exerts several effects on separase. First, PTTG1 inhibits separase activity. Second, binding of PTTG1 stabilizes separase. Finally, PTTG1 activates separase. This was apparent in the presence of PTTG1^H134R, which lost the inhibitory activity but retained the activating activity. In agreement with this, PTTG1 lacking H¹³⁴ induced a similar mitotic block as separase overexpression. Moreover, I found that activation of separase may in part involve the C-terminus of PTTG1. Collectively, these results suggest that the H¹³⁴-containing region has a strong affinity for separase and is involved in inhibiting separase. On the other hand, the C-terminus of PTTG1 is involved in activating separase and has a relatively weaker binding affinity for separase. These results for the first time demonstrated the activating and inhibitory functions of securin on separase are separable. These results also illustrate an interesting example of using an inactive isoform to investigate the functions of an active isoform.