Ordering the phosphorylation of cyclin-dependent kinase Cdk1 substrates in the cell cycle
Date
2021-03-23
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Abstract
Rakutsükkel on sündmuste jada, mis tagab rakkude jagunemise. Veatuks jagunemiseks peavad erinevad sündmused toimuma kindlas järjekorras ja sünkroniseeritult, nii et rakuline materjal esmalt kahekordistatakse ja seejärel jaotatakse kahe tütarraku vahel. Näiteks, paralleelselt DNA replikatsiooniga toimuvad ka tsentrosoomi duplikatsioon ja muutused raku metabolismis. Rakutsüklit koordineerivad tsükliinist sõltuvad kinaasid (CDK-d), mis fosforüleerimise kaudu reguleerivad sadade valkude aktiivsust. Kusjuures, muutused Cdk1 aktiivsuses on vajalikud ja piisavad, et käivitada DNA replikatsioon ja mitoos.
CDK on katalüütiliselt aktiivne vaid kompleksis tsükliiniga ning erinevates rakutsükli faasides on avaldunud erinevad tsükliinid, mistõttu tekivad erinevad tsükliin-CDK kompleksid. CDK keskne roll rakutsüklis on pannud aluse küsimusele, millised mehhanismid võimaldavad sel kinaasil erineva ajastusega fosforüleerida sadu substraate. Selle küsimuse vastuseks on pakutud kaks mudelit. Rakutsükli jooksul CDK aktiivsus tõuseb ning kvantitatiivse mudeli kohaselt fosforüleerib CDK spetsiifilised valgud erinevatel kinaasi aktiivsuse tasemetel. Tsükliini spetsiifilisuse mudeli kohaselt aga suunavad erinevad tsükliinid CDK kompleksi kindlaid substraate fosforüleerima.
Käesolevas töös uuriti Cdk1 kompleksi substraatide äratundmise mehhanisme eesmärgiga mõista, kuidas need interaktsioonid võimaldavad kinaasil Cdk1 erineva ajastusega sadu märklaudvalke fosforüleerida. Leiti, et substraatide fosforüleerimise ajastamisel on olulisel kohal nii CDK tõusev aktiivsus kui ka spetsiifilised tsükliin-substraat interaktsioonid. Lisaks kirjeldati uued tsükliin-substraat seondumismotiivid, mis on spetsiifilised kas S-, G2- või M-faasi tsükliinidele. Töös näidati, et erinevate motiivide, nagu fosforüleerimis- ja seondumismotiivide, mustrid määravad CDK substraatvalkude fosforüleerimise ajastuse rakutsüklis.
Cell cycle is a series of events that guarantees the reproduction of cells. A flawless cell cycle requires a specific order of events, whereby the cellular contents are first duplicated and later segregated. Further, various events must be synchronized, for example DNA replication is coordinated with centrosome duplication and changes in cell metabolism. This is achieved by the action of cyclin-dependent kinases (CDKs) that phosphorylate and regulate the activity of hundreds of proteins. Interestingly, oscillation in the activity of Cdk1 is necessary and sufficient to initiate DNA replication and mitosis. CDKs require binding of cyclin proteins for enzymatic activity. Different waves of cyclins are expressed in G1, S, G2 and M phases, resulting in the formation of distinct cyclin-CDK complexes in different cell cycle stages. Two models have been proposed to answer the question of which mechanisms direct the timely phosphorylation of Cdk1 targets. The quantitative model states that specific proteins are phosphorylated at distinct CDK activity thresholds and thus the order of events is governed by the increase in kinase activity during the cell cycle. Alternatively, the cyclin specificity model proposes that different cyclins direct CDK to phosphorylate stage-specific substrates. In this study, the substrate targeting interactions of the Cdk1 complex were studied with the aim to understand the mechanisms that enable Cdk1 to differentially phosphorylate hundreds of proteins. We found that CDK thresholds can be encoded both based on the increase in CDK activity in the cell cycle and based on cyclin-specific substrate targeting. We identify novel cyclin docking motifs that enable specific phosphorylation by S, G2 or M phase CDK complexes. Importantly, we show that the pattern of linear motifs, including phosphorylation sites and docking motifs, can determine the timing of CDK substrate phosphorylation throughout the cell cycle.
Cell cycle is a series of events that guarantees the reproduction of cells. A flawless cell cycle requires a specific order of events, whereby the cellular contents are first duplicated and later segregated. Further, various events must be synchronized, for example DNA replication is coordinated with centrosome duplication and changes in cell metabolism. This is achieved by the action of cyclin-dependent kinases (CDKs) that phosphorylate and regulate the activity of hundreds of proteins. Interestingly, oscillation in the activity of Cdk1 is necessary and sufficient to initiate DNA replication and mitosis. CDKs require binding of cyclin proteins for enzymatic activity. Different waves of cyclins are expressed in G1, S, G2 and M phases, resulting in the formation of distinct cyclin-CDK complexes in different cell cycle stages. Two models have been proposed to answer the question of which mechanisms direct the timely phosphorylation of Cdk1 targets. The quantitative model states that specific proteins are phosphorylated at distinct CDK activity thresholds and thus the order of events is governed by the increase in kinase activity during the cell cycle. Alternatively, the cyclin specificity model proposes that different cyclins direct CDK to phosphorylate stage-specific substrates. In this study, the substrate targeting interactions of the Cdk1 complex were studied with the aim to understand the mechanisms that enable Cdk1 to differentially phosphorylate hundreds of proteins. We found that CDK thresholds can be encoded both based on the increase in CDK activity in the cell cycle and based on cyclin-specific substrate targeting. We identify novel cyclin docking motifs that enable specific phosphorylation by S, G2 or M phase CDK complexes. Importantly, we show that the pattern of linear motifs, including phosphorylation sites and docking motifs, can determine the timing of CDK substrate phosphorylation throughout the cell cycle.
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Keywords
cell cycle, cyclin dependent kinases, cyclins, proteins, phosphorylation