A geneticist has devised a clever strategy to study protein translocation into t

Question

A geneticist has devised a clever 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 operated via slightly different translocation mechanisms. Using genetic engineering, she has fused the first signal sequence to a protein whose presence in the cytosol s absolutely necessary for survival in the selective medium, but which is inactive when in the ERR. In the same cells, she has fused the second signal sequence to a toxic protein whose presence in the cytosol leads to cell death but is harmless when in the ER. Whereas wild-type cells die by cell lysis upon production of both 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 ER membrane translocation. The normal protein products of these mutated genes are probably: involved in the transport of proteins with the first signal sequence but not the second one involved in the general transport of proteins into the ER, regardless of the type of signal sequence involved in the transport of proteins with a novel signal sequence (i.e. neither the first signal sequence nor the second one). involved in the transport of proteins with the second signal sequence but not the first one.

Explanation / Answer

A. involved in the transport of proteins with the first signal sequence but not the second one.

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