You are a scientist in a genetics lab studying yeast cells. Your group looks at protein translocation to the ER, and has identified two signal sequences of interest. Your job is to determine whether they use the same transport mechanism. You create a transgene for a fusion of sequence A and an essential protein (which is functional in the yeast cytoplasm, but not when it is in the ER). You also create a fusion protein that tags a cytosolic toxin with sequence B; this toxic protein is inert when it is localized in the ER. When you transform wild-type yeast with both transgenes simultaneously, the cells lyse and die. You screen your transgenes against a library of mutant yeast, and find strains that survive coexpression of both constructs. The mutant strains all have mutations that cause loss-of-function in a membrane translocation protein. You conclude that the mutated protein is probably... A. involved in the transport of proteins with a novel signal sequence (i. e. neither the first signal sequence nor the second one). B. involved in the general transport of proteins into the ER, regardless of the type of signal sequence. C. involved in the transport of proteins with the first signal sequence but not the second one. D. involved in the transport of proteins with the second signal sequence but not the first one.