To study protein localization signals you have engineered yeast strains that exp

Question

To study protein localization signals you have engineered yeast strains that express the indicated fusion proteins below.

A) A PMP fusion protein with an amino terminal hydrophobic signal sequence and a positively charged carboxy terminal signal. PMP (Protein Misfolding Peptide) is toxic to yeast only if iaccumulates in the ER. Predict if expression of the PMP fusion protein will impact yeast viability. Be sure to justify your answer in the context of protein localization.

B) Cathepsain X with an amino terminal amphiphilic alpha helix sequence and a carboxy terminal KDEL. When present in growth media, chaotropic salts disrupt protein folding and impair yeast growth. If cathepsain X is localized to the ER lumen, it facilitates protein folding and allows yeast to grow in the presence of these salts. Predict how expression of cathepsainX fusion protein will influence yeast growth when cells are incubated in media containing chaotropic salts. Be sure to justify your answer in the context of protein localization.

Explanation / Answer

* Proteins with hydrophobic amino terminal are destined to ER. As the given protein has an amino terminal hydrophobic signal sequence, it will be transported into the ER. As the fucntion of this proteins is misfolding of proteins, it will affect the functions of newly synthesized proteins by misfolding them. The protein will be toxic to yeast cells.

* Chaotropic salts disrupt protein folding mainly by disturbing the hydrophobic interactions. The hydrophobic interactions stabilize a proteins strucutre, while covalent bonds are the initial lead takers. Chaotropic salts expose the otherwise hidden hydrophobic regions. The proteins can now pass through the ER membrane easily due to the exosed hydrophobic regions. Cathepsain X localized in ER helps to fold the proteins properly by establishin hydrophobic interactions.

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