Question

1. Inspired by Lee Hartwell's Nobel Prize (2001), you decide to study cell cycle regulation in yeast. You know that the expression of a cell cycle-associated kinase, Cdk24p, changes throughout the cell cycle, being very high in G 1/S and low in G2/M. Nevertheless, mRNA levels for CDK24 are constant throughout the cell cycle. You decide to study how this regulation occurs.

You isolate a temperature sensitive mutant cell linethat fails to divide at the non-permissive temperature and notice that Cdk24p expression is not downregulated after S phase at the non­ permissive temperature - it remains constitutively high. Using standard techniques, you isolate the gene that is mutated in the temperature-sensitive cell line; it is a previously undescribed gene, Cdc927, encoding a protein with two "zinc fingers" that, upon comparison to the database, fall into the category of a "ring" domain--similar to those seen in E3 ubiquitin ligases. The mutant that you isolated has a cdc927 mutation altering an arginine near a conserved cysteine within the ring domain.

Interestingly, you find that in cells with this cdc927 mutation, a panel of 8 cell cycle associated proteins fail to be down-regulated after S phase at the non­permissive temperature. mRNA levels for all of these proteins are constant throughout the cell cycle in wild-type cells. Moreover, temperature-sensitive mutants of proteasome subunits also produce the same phenotype as the cdc927 mutant, i.e. stabilization of all 8 proteins.

1. 1. In which stage of the cell cycle does Cdc927p likely act? Why
2. 2. Given these data, what you know about zinc fingers, and what you know about cell cycle regulation, propose a hypothesis for a putative function of Cdc927p.  To what family of proteins does Cdc927p likely belong?

Answer 1

Aus: E3 ligases are play a as of cyclins, as well agradation 3 ligases are play a role in cell cycle a " helps in degradation of cyclins, as we Cyclin dependent kinase inhibitor proteins E3 ubiquitin ligases helps in the reg ne p53 and retinoblastoma protein (PRB) per ligeses helps in of the regulation way. Ansi-b:- Cell each stage cycle of regulat cell " Cycle regulation is the process in which stage of cell cycle is monitoring control by different check points in each phase of Cell cycle. For example, if DNA is not propenly synthesis in so phase, then check points detects this defect and then cell cycle stop at s-phase until DNA gets properly synthesis. Cdc 121p belongs to the ubiquitin ligase Families of protein →

to cell cycle and cycle assest G phase Cellular contents, excluding the choomosomes, are - duplicated S phase After completion of Mitosis phase Each of the 46 daughter cells are chromosomes is produced duplicated by the G phase The cell "double checks" the duplicated chromosomes for esmor, making any needed repairs stages of cell cycle :- G SGM 7 VG Mitosis. Inteuphase Zinc finger :- Zinc finger could be use to recognize very specific DNA sequences. Zinc Jinger is a fingen shaped Told in a protein that permits or help in interaction with DNA or RNA- That fold in zinc fingey is due the binding of specific amino acids like cysteines and histidines in the protein to a zinc atom.
Ans- a. Cdc927p likely act on Mitosis (M) phase of cell cycle because it is similar as E3 ubiquitin ligase and this ligase act on M-phase of cycle for targeting the substrate for destruction by proteosome. This ligase also contain Anaphase promoting complex (APC) and Skp1-Cullin-F-box protein complex (SCF complex). APC/C main function is helps in transition from metaphase to Anaphase by specific protein degradation.