You set up an in vitro translation system containing the ent…
You set up an in vitro translation system containing the entire translation machinery but missing any component of the endoplasmic reticulum (ER) targeting machinery. To this system, you can add mRNAs encoding either a 20 kDa secretory protein or a 20 kDa 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 ER-derived microsomes, as indicated below. After separating the protein products by SDS-PAGE, and visualizing the radioactivity by autoradiography, you obtain the below 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 is the secretory protein ? Which of the reactions omitted SRP ?