Male cockroaches with mutations that strongly decrease the function of an RTK called RTKX are oblivious to the charms of their female comrades. This particular RTK binds to a small molecule secreted by sexually mature females

Question

Most males carrying loss-of-function mutations in the gene for Ras protein are also unable to respond to females. You have just read a paper in which the authors describe how they have screened cockroaches that are mutant in RTKX for additional mutations that partly restore the ability of males to respond to females. These mutations decrease the function of a protein that the authors call Z. Which of the following types of protein could Z be? Explain your answer.
(a) a protein that activates the Ras protein by causing Ras to exchange GDP for GTP
(b) a protein that stimulates hydrolysis of GTP by the Ras protein
(c) an adaptor protein that mediates the binding of the RTKX to the Ras protein
(d) a transcriptional regulator required for the expression of the Ras gene

Answer 1

(b) Mutations that increase the activity of Ras should mimic the effect of stimulating RTKX in a receptor-independent fashion. Because the intracellular concentration of GTP is higher than that of GDP, some proportion of the Ras molecules is expected to be GTP-bound and active; ridding the cells of a protein that stimulates GTP hydrolysis will increase this pool of active Ras. Mutants that cannot stimulate Ras to exchange GDP for GTP will have the same phenotype as mutants lacking Ras [choice (a)], as will mutants lacking a transcriptional regulator required for expression of the Ras gene [choice (d)]. Defects in an adaptor protein that mediates the binding of receptor X to Ras will have no further effect on a mutant already lacking the receptor [choice (c)].