Please Answer both parts (A&B). Question 1 is multiple choice. 1 ) Suppose you g
ID: 65429 • Letter: P
Question
Please Answer both parts (A&B). Question 1 is multiple choice.
1 ) Suppose you go into your lab and engineer a chimeric Ras protein in which the N-terminal 20 amino acids of Ras are removed and replaced by the N-terminal 20 amino acids of insulin. What is the final destination to which this protein will be trafficked if expressed in a mouse pancreatic cell? _____
a. cytoplasm
c. nucleus
e. late endosome
b. extracellular fluid d. ER lumen
f. cell surface (integral to the plasma membrane)
B) To recap, your chimeric Ras protein has its N-terminal 20 amino acids from insulin and the remainder of its amino acid sequence from Ras. Your annoying scientific arch-rival from another university publishes a “letter to the editor” predicting this chimeric protein will localize to the inner face of the plasma membrane because it still has the C-terminal CAAX motif of Ras, which he claims will be recognized and lipid-modified by farnesyl:protein transferase in the cytoplasm. Write a brief rebuttal letter in which you use your knowledge of cell biology to puncture his preposterous postulate.
Explanation / Answer
A) d. ER lumen. Since insulin hormone has a 24-residue signal peptide which directs the nascent polypeptide chain to the rough endoplasmic reticulum (RER), the chimeric Ras will localize into ER.
B) Since translation takes place from the 5' to 3' direction in an mRNA and translation from N to C terminus, the N terminal will be recognized first by the Signal Recognition Particle (SRP) on the ER membrane and will drive the whole protein into the ER instead of inner face of the plasma membrane.