1) The autoradiogram shown below shows the results of the experiment in which each translation reaction was carried out either in the presence (+) or the absence (?) of microsomes. Based on the gel mobility of peptides synthesized in the presence or absence of microsomes,

a) how long must the prolactin nascent chain be in order for the prolactin signal peptide to enter the ER lumen and be cleaved by signal peptidase? (Note that microsomes carry significant quantities of SRP bound to the membranes.)

b) Why are there two bands in the 130+ lane?

2) 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 ? long; a membrane bilayer is about 50 ? thick; in polypeptides with an ?-helical conformation, one residue extends 1.5 ?, whereas in fully extended polypeptides, one residue extends about 3.5 ?.

Transcribed Image Text:

Size of labeled polypeptide 50 + 70 90 110 Size of mRNA (in codons) 130 150 A B Size of labeled polypeptide 50 + 70 90 110 Size of mRNA (in codons) 130 150 A B

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1 a The prolactin nascent chains must be 130 codons long inorder to enter the ER lumen and get cleav... View the full answer