Question

Suppose you find four new proteins, that you have named proteins A, B, C, and D. You have determined that they regulate the cell cycle, so they must be either cyclins or CDKs. You are interested in figuring out which of the four proteins are cyclins, and which are CDKs. In an experiment using budding yeast, you find that protein A is found at higher concentrations when the bud on the yeast cells is small, and is complexed with protein C. You also determine that protein D is found at higher concentrations when the bud on the yeast cells is large, and is in complex with Protein B. Proteins B and C remain at constant concentration at all times in the yeast cells.

Based off of the information above, which of the four proteins are cyclins, and which of the four proteins are CDKs? Which part of the cell cycle could each of these proteins be controlling?

To obtain a more clear idea of which part of the cell cycle each protein controls, you decide to do some further testing. You add the ubiquitin ligase SCF and see that protein A degrades, but proteins B, C, and D remain intact. Conversely, when you add the ubiquitin ligase APC/C, protein D is degraded, but proteins A, B, and C remain intact. Based off these results, which part of the cell cycle does each protein complex regulate? Does the degradation pattern with addition of ubiquitin ligase in part B match your answer in part A?

Answer 1

# Protein B and C are cyclin dependent kinases. That is why they are constant through out the cell cycle.

- Protein A controls the G1/S phase. That is why this protein was very high when the cell was in small size. It was promoting the cell to S phase. Degradation of protein A with ubiquitin ligase SCF confirms that the protein is cyclin which involves in the G1/S phase.

- Protein D controls the G2/M phase. This protein is high when the cell size is big because, after S phase, all the molecules are duplicated including DNA. Therefore, protein D involves in the control of G2/M phase. Degradation of protein D by ubiquitin ligase APC/C confirms that this protein promotes mitosis.