Critical Differences between Isoforms of Securin Reveal Mechanisms of Separase Regulation

Abstract

Sister chromatid separation depends on the activity of separase, which in turn requires the proteolysis of its inhibitor, securin. It has been speculated that securin also supports the activation of separase. In this study, we found that PTTG1 was the major securin isoform expressed in most normal and cancer cell lines. Remarkably, a highly homologous isoform called PTTG2 was unable to interact with separase. Using chimeras between PTTG1 and PTTG2 and other approaches, we pinpointed a single amino acid that accounted for the loss of securin function in PTTG2. Mutation of the homologous position in PTTG1 (H¹³⁴) switched PTTG1 from an inhibitor into an activator of separase. In agreement with this, PTTG1 lacking H¹³⁴ was able to trigger premature sister chromatid separation. Conversely, introduction of H¹³⁴ into PTTG2 is sufficient to allow it to bind separase. These data demonstrate that while the motif containing H¹³⁴ has a strong affinity for separase and is involved in inhibiting it, another domain(s) is involved in activating separase and has a weaker affinity for it. Although PTTG2 lacks securin function, its differences from PTTG1 provide evidence of independent inhibitory and activating functions of PTTG1 on separase.

View publisher note:

Article of Significant Interest Selected from This Issue by the Editors

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.00057-13.

ACKNOWLEDGMENTS

We thank Ken Ma for the APC/C reporter cell line, as well as help with preparing lysates from different cell lines, and Wing Yu Man for technical assistance.

This work was supported in part by Research Grants Council grants AOE-MG/M-08/06 and HKU7/CRG/09.

We declare that we have no conflict of interest.