The movement of a ciliated protozoan is controlled by a protein called RacerX. When this protein binds to another protein found at the base of the cilia, it stimulates the cila to beat faster and the protozoan to swim faster. This ciliar protein, Speed, can be phosphorylated and can only bind to RacerX in its phosphorylated form. You have identified the threonine residue at which Speed is phosphorylated and changed it to an alanine residue. How would you expect the mutant protozoan to behave?

A. Swims fast occasionallyB. Always swims fastC. Never swims fastD. Switches rapidly back and forth between fast and slow swimmingE. Cannot move at all

C. Never swims fast

Explanation:

RacerX and Speed protein need to bind to each other for the cilia to beat faster which would make the protozoan swim faster. However, Speed needs to be phosphorylated to be able to bind to RacerX. Thus, phosphorylation of Speed is necessary for the fast movement of cilia.

Threonine residue is phosphorylated in Speed protein. It occurs on its side chain through the formation of phosphoester bond. Apart from threonine, phosphorylation can take place on serine and tyrosine side chains again through the formation of phosphoester bond. It can also occur on histidine, lysine and arginine by formation of phosphoramidate bonds.

Since, alanine can not form any of these bonds it can not be phosphorylated. Hence when threonine is substituted with alanine in Speed protein, it can not be phosphorylated. Racer X wont be able to bind to Speed and cilia wont beat faster. As a result the protozoan will never swim fast.