Identification of a Vertebrate Sister-Chromatid Separation Inhibitor Involved in Transformation and Tumorigenesis

Abstract

A vertebrate securin (vSecurin) was identified on the basis of its biochemical analogy to the Pds1p protein of budding yeast and the Cut2p protein of fission yeast. The vSecurin protein bound to a vertebrate homolog of yeast separins Esp1p and Cut1p and was degraded by proteolysis mediated by an anaphase-promoting complex in a manner dependent on a destruction motif. Furthermore, expression of a stable Xenopus securin mutant protein blocked sister-chromatid separation but did not block the embryonic cell cycle. The vSecurin proteins share extensive sequence similarity with each other but show no sequence similarity to either of their yeast counterparts. Human securin is identical to the product of the gene called pituitary tumor-transforming gene (PTTG), which is overexpressed in some tumors and exhibits transforming activity in NIH 3T3 cells. The oncogenic nature of increased expression of vSecurin may result from chromosome gain or loss, produced by errors in chromatid separation.

Cell cycle regulated expression and phosphorylation of hpttg proto-oncogene product

“…Although we cannot discard that these sites could be phosphorylated at low level by Cdc2, our S 165 A mutant almost completely abrogated the in vitro phosphorylation by this kinase, indicating that this optimal consensus site is the main phosphorylation site. Interestingly this serine is conserved in rat and mouse PTTG, however it is not present in the Xenopus PTTG sequence (Zou et al, 1999). The possibility that the phosphorylation of other serine in Xenopus have the same role that phosphorylation of Ser 165 in human PTTG, needs further investigation.…”

“…These observations further raise the possibility that hpttg may be involved in regulating progression through the cell cycle and, more speci®cally, through the M phase. During the revision of this manuscript, Zou et al (1999) reported that PTTG encodes a securin, a protein which inhibits sister-chromatid separation by binding to separin. Our results are in total agreement with this function of hPTTG.…”

“…Failure to normally regulate hPTTG level could lead to the uncontrolled proliferation that is a hallmark of cancer, and could explain the in vitro and in vivo transforming abilities of hpttg. On the basis of its function as a securin, Zou et al (1999) propose that PTTG tumorigenic activity is the result of aneuploidy caused by defects in the sister-chromatid separation. On the other hand, our results predict that any stimulus or genetic alteration leading to cell proliferation would lead to high steady-state hPTTG level.…”

“…According to Zou et al (1999), PTTG is bound to separin until the end of metaphase when PTTG is degraded by anaphase-promoting complex-mediated proteolysis. An interesting possibility, that is currently being explored in our laboratory, is that phosphorylation by Cdc2 could regulate the interaction of hPTTG with separin or its degradation.…”

Cohesin and chromatin organisation

“…At anaphase the ubiquitin-ligase anaphase-promoting complex (APC) induces the degradation of Securin, an inhibitor of the protease Separase, which in turn cleaves the Rad21 subunit of cohesin [42][43][44]. Sister chromatids are now free to follow the pull of the mitotic spindles and to segregate towards the newly forming daughter cells.…”