Suppose that you discover cells containing a mutation in a second protein and learn that this mutation also prevents mitotic cyclin from being degraded at the onset of mitosis, even when mitotic cyclin is normal. The degradation of other proteins appears to proceed normally in such cells. A mutation in what kind of protein might explain such results

Mutations affecting proteins involved in the ubiquitination pathway

Explanation:

The cyclins represent a group of proteins capable of regulating the progression of the cell cycle via activation of cyclin-dependent kinases (CDKs). Cyclins are degraded by ubiquitination, a posttranslational modification that triggers the exit of mitosis by "marking" cyclin proteins for their subsequent degradation in the proteasome. The proteasome is a macromolecular complex required for the degradation of proteins in the cell. Then consequently the mutations affecting both secondary proteins required during ubiquitination and those involved in the functioning of the proteasome complex might prevent the degradation of cyclin proteins.

the supermassive black hole is in the middle of the galaxy.