Cancer/Testis Antigen PASD1 Silences the Circadian Clock.

The circadian clock orchestrates global changes in transcriptional regulation on a daily basis via the bHLH-PAS transcription factor CLOCK:BMAL1. Pathways driven by other bHLH-PAS transcription factors have a homologous repressor that modulates activity on a tissue-specific basis, but none have been identified for CLOCK:BMAL1. We show here that the cancer/testis antigen PASD1 fulfills this role to suppress circadian rhythms. PASD1 is evolutionarily related to CLOCK and interacts with the CLOCK:BMAL1 complex to repress transcriptional activation. Expression of PASD1 is restricted to germline tissues in healthy individuals but can be induced in cells of somatic origin upon oncogenic transformation. Reducing PASD1 in human cancer cells significantly increases the amplitude of transcriptional oscillations to generate more robust circadian rhythms. Our results describe a function for a germline-specific protein in regulation of the circadian clock and provide a molecular link from oncogenic transformation to suppression of circadian rhythms.

Conservation of mechanisms controlling entry into mitosis: budding yeast wee1 delays entry into mitosis and is required for cell size control.

BACKGROUND: In fission yeast, the Wee1 kinase delays entry into mitosis until a critical cell size has been reached; however, a similar role for Wee1-related kinases has not been reported in other organisms. SWE1, the budding yeast homolog of wee1, is thought to function in a morphogenesis checkpoint that delays entry into mitosis in response to defects in bud morphogenesis. RESULTS: In contrast to previous studies, we found that budding yeast swe1 Delta cells undergo premature entry into mitosis, leading to birth of abnormally small cells. Additional experiments suggest that conditions that activate the morphogenesis checkpoint may actually be activating a G2/M cell size checkpoint. For example, actin depolymerization is thought to activate the morphogenesis checkpoint by inhibiting bud morphogenesis. However, actin depolymerization also inhibits bud growth, suggesting that it could activate a cell size checkpoint. Consistent with this possibility, we found that actin depolymerization fails to induce a G2/M delay once daughter buds pass a critical size. Other conditions that activate the morphogenesis checkpoint block bud formation, which could also activate a size checkpoint if cell size at G2/M is monitored in the daughter bud. Previous work reported that Swe1 is degraded during G2, which was proposed to account for failure of large-budded cells to arrest in response to actin depolymerization. However, we found that Swe1 is present throughout G2 and undergoes hyperphosphorylation as cells enter mitosis, as found in other organisms. CONCLUSIONS: Our results suggest that the mechanisms known to coordinate entry into mitosis in other organisms have been conserved in budding yeast.

A phosphatase threshold sets the level of Cdk1 activity in early mitosis in budding yeast.

Entry into mitosis is initiated by synthesis of cyclins, which bind and activate cyclin-dependent kinase 1 (Cdk1). Cyclin synthesis is gradual, yet activation of Cdk1 occurs in a stepwise manner: a low level of Cdk1 activity is initially generated that triggers early mitotic events, which is followed by full activation of Cdk1. Little is known about how stepwise activation of Cdk1 is achieved. A key regulator of Cdk1 is the Wee1 kinase, which phosphorylates and inhibits Cdk1. Wee1 and Cdk1 show mutual regulation: Cdk1 phosphorylates Wee1, which activates Wee1 to inhibit Cdk1. Further phosphorylation events inactivate Wee1. We discovered that a specific form of protein phosphatase 2A (PP2A(Cdc55)) opposes the initial phosphorylation of Wee1 by Cdk1. In vivo analysis, in vitro reconstitution, and mathematical modeling suggest that PP2A(Cdc55) sets a threshold that limits activation of Wee1, thereby allowing a low constant level of Cdk1 activity to escape Wee1 inhibition in early mitosis. These results define a new role for PP2A(Cdc55) and reveal a systems-level mechanism by which dynamically opposed kinase and phosphatase activities can modulate signal strength.

The structure of a monomeric mutant Cks protein reveals multiple functions for a conserved hinge-region proline.

Cks (cyclin-dependent kinase subunit) proteins are essential eukaryotic cell cycle regulatory proteins that physically associate with cyclin-dependent kinases (Cdks) to modulate their activity. Cks proteins have also been studied for their ability to form domain-swapped dimers by exchanging ß-strands. Domain swapping is mediated by a conserved ß-hinge region containing two proline residues. Previous structural studies indicate that Cks in its dimer form is unable to bind Cdk, suggesting that the monomer-dimer equilibrium of Cks may have an effect on Cks-mediated Cdk regulation. We present the crystal structure of a proline-to-alanine mutant Saccharomyces cerevisiae Cks protein (Cks1 P93A) that preferentially adopts the monomer conformation but surprisingly fails to bind Cdk. Comparison of the Cks1 P93A structure to that of other Cks proteins reveals that Pro93 is critical for stabilizing a multiple ß-turn structure in the hinge region that properly positions an essential Cdk-binding residue. Additionally, we find that these ß-turn formations, conserved in Cks homologs, have implications for the mechanism and preferentiality of strand exchange. Together, our observations suggest that the conservation of Cks hinge-region prolines reflects their functions in forming a Cdk binding interface and that the ability of these prolines to control partitioning between monomer and dimer is a consequence of the ß-turn networks within the hinge.

Cdk1-dependent regulation of the mitotic inhibitor Wee1.

The Wee1 kinase phosphorylates and inhibits cyclin-dependent kinase 1 (Cdk1), thereby delaying entry into mitosis until appropriate conditions have been met. An understanding of the mechanisms that regulate Wee1 should provide new insight into how cells make the decision to enter mitosis. We report here that Swe1, the budding-yeast homolog of Wee1, is directly regulated by Cdk1. Phosphorylation of Swe1 by Cdk1 activates Swe1 and is required for formation of a stable Swe1-Cdk1 complex that maintains Cdk1 in the inhibited state. Dephosphorylation of Cdk1 leads to further phosphorylation of Swe1 and release of Cdk1. Thus, Cdk1 both positively and negatively regulates its own inhibitor. Regulation of the Swe1-Cdk1 complex is likely to play a critical role in controlling the transition into mitosis.