Imagine that you are evaluating the early steps in translocation and processing of the secretory protein prolactin.
Question:
a. The autoradiogram depicted below shows the results of an experiment in which each translation reaction was carried out either in the presence (+) or the absence (-) of microsomal membranes. Based on the gel mobility of pcptides synthesized in the presence or absence of microsomes, deduce how long the prolactin nascent chain must be in order for the prolactin signal peptide to enter the ER lumen and to be cleaved by signal peptidase.
b. Given this length, what can you conclude about the conformational state(s) of the nascent prolactin polypeptide when it is cleaved by signal peptidase? The following lengths will be useful for your calculation: the prolactin signal sequence is cleaved after amino acid 31; the channel within the ribosome occupied by a nascent polypeptide is about 150 A long; a membrane bilayer is about 50 A thick; in polypeptides with an a-helical conformation, one residue extends 1.5 A, whereas in fully extended polypeptides, one residue extends about 3.5 A.
c. The experiment described in part {a) is carried out in an identical manner except that microsomal membranes are not present during translation but are added after translation is complete. In this case none of the samples shows a difference in mobility in the presence or absence of microsomes. What can you condude about whether prolactin can be translocated into isolated microsomes post-translationally?
d. In another experiment, each translation reaction was carried out in the presence of microsomes, and then the microsomal membranes and bound ribosomes were separated from free ribosomes and soluble proteins by centrifugation. For each translation reaction, both the total reaction (T) and the membrane fraction (M) were resolved in neighboring gel lanes. Based on the amounts of labeled polypeptide in the membrane fractions in the autoradiogram depicted below, deduce how long the prolactin nascent chain must be in order for ribosomes engaged in translation to engage the SRP and thereby become bound to microsomal membranes.
Step by Step Answer:
Molecular Cell Biology
ISBN: 978-1429234139
7th edition
Authors: Harvey Lodish, Arnold Berk, Chris A. Kaiser, Monty Krieger, Anthony Bretscher, Hidde Ploegh, Angelika Amon, Matthew P. Scott