You set up an in vitro translation system containing the entire translation machinery but devoid of any component of the endoplasmic reticulum (ER) targeting machinery. To this system, you can add mRNA encoding either a 20 kD secretory protein or a 20 kD cytosolic protein. You perform in vitro translation in the presence of radioactively labeled methionine, with or without the addition of saturating amounts of SRP or microsomes, as indicated below. After separating the protein products by SDS-PAGE, and visualizing the radioactivity by autoradiography, you obtain the following results. The presence or absence of each component in the reaction is indicated at the top of the corresponding lane(s) by + and -, respectively. The numbers on the left indicate the apparent molecular mass (×1000) of spots on the gel. Which protein (X or Y) is the secretory protein? Which of the reactions (1 or 2) contained SRP?
A) X; 1
B) X; 2
C) Y; 1
D) Y; 2

Question

You set up an in vitro translation system containing the entire translation machinery but devoid of any component of the endoplasmic reticulum (ER) targeting machinery. To this system, you can add mRNA encoding either a 20 kD secretory protein or a 20 kD cytosolic protein. You perform in vitro translation in the presence of radioactively labeled methionine, with or without the addition of saturating amounts of SRP or microsomes, as indicated below. After separating the protein products by SDS-PAGE, and visualizing the radioactivity by autoradiography, you obtain the following results. The presence or absence of each component in the reaction is indicated at the top of the corresponding lane(s) by + and -, respectively. The numbers on the left indicate the apparent molecular mass (×1000) of spots on the gel. Which protein (X or Y) is the secretory protein? Which of the reactions (1 or 2) contained SRP? 
A) X; 1 
B) X; 2 
C) Y; 1 
D) Y; 2

Quizplus · Accepted Answer

The cytosolic protein is unaffected by the presence or absence of microsomes or SRP. Addition of both SRP and microsomes to the reaction (lane 3) results in the microsomal import of the secretory protein and the cleavage of its signal peptide, while addition of SRP alone (lane 1) inhibits peptide elongation beyond a short polypeptide length, corresponding to the length of the signal sequence plus the fragment that is still in the ribosomal exit tunnel.

You Set Up an in Vitro Translation System Containing the Entire