A geneticist has devised a strategy to study protein translocation into the endoplasmic reticulum (ER) in yeast cells. She is interested in two different signal sequences that are thought to operate via slightly different translocation mechanisms. Using genetic engineering, she has fused the first signal sequence to a protein whose cytosolic expression is absolutely necessary for cell survival in the selective medium, but is inactive when in the ER. In the same cell, she has also fused the second signal sequence to a toxic protein whose cytosolic expression leads to cell lysis but is harmless when in the ER. Whereas wild-type cells undergo lysis upon the expression of these fusion proteins, she has been able to identify viable mutants, each of which has a loss-of-function mutation in a gene encoding a protein involved in membrane translocation. The products of these genes are probably ...
A) involved in the general transport of proteins into the ER, regardless of the type of signal sequence.
B) involved in the transport of proteins with the first signal sequence but not the second one.
C) involved in the transport of proteins with the second signal sequence but not the first one.
D) involved in the transport of proteins with a novel signal sequence (i.e. neither the first signal sequence nor the second one).

Question

Quizplus · Accepted Answer

This genetic screen identifies mutations that interfere with the ER targeting of the vital protein (which has been fused to the first signal sequence) while keeping the ER targeting of the toxic protein (with the second signal sequence) almost intact, resulting in cell viability. Therefore, the mutations are expected to affect genes that encode proteins involved in the first pathway but not the second.

A Geneticist Has Devised a Strategy to Study Protein Translocation