You perform a second experiment, in which you add SRP to your cell-free protein-

ID: 161597 • Letter: Y

Question

You perform a second experiment, in which you add SRP to your cell-free protein-synthesizing system, and find that translation stops after a polypeptide about 70 amino acids long has been produced. How can you explain this result? Can you think of any purpose this phenomenon might serve for the cell?

Drag the terms on the left to the appropriate blanks on the right to complete the sentences. Not all items will be used.

ER signal binds to the of the newly forming sequence polypeptide chain and halts protein synthesis, thereby preventing the SRP chain from growing beyond 70 amino acids. Inside cells, this blockage is binds the normally maintained until the SKL sequence to the Without this blockage, a Complete preprolactin signal peptidase chain might be produced by cells and released into the cytosol rather than being transported into the stop-transfer sequence start-transfer sequence KDEL sequence GTP polypeptide ribosome ER lumen ER membrane

Explanation / Answer

Answer

After translation, the synthesized protein is modified in the endoplasmic reticulum for post translational modifications to occur. At the N-terminus of the protein, there is a signal sequence which directs it to the endoplasmic reticulum. This confirms that the signal is first synthesized in the ribosome and released from it, prior to the release of entire protein. This sequence is recognized by specific RNA-protein complex. This complex is called signal recognition particle (SRP).

This SRP binds to the signal sequence and directs the protein to the endoplasmic reticulum. After the modifications, these proteins are targeted to the Golgi vesicles, meant for secretion. If there is a sequence of KDEL(Lysine-Aspartate-Glutamate-Leucine) in the protein, they are redirected to the endoplasmic reticulum.

If the protein is membrane bound protein, the protein modified in the lumen of endoplasmic reticulum makes the alpha helix of the folded protein insert in the membrane and the carboxy terminus in the cytosol. This positioning of the protein in the membrane is assisted by the sequence called stop-transfer sequence.

From the explanation, the blanks can be filled as follows:

SRP binds to the ER signal sequence of the newly forming polypeptide chain and halts protein synthesis, thereby preventing the chain from growing beyond 70 amino acids. Inside cells, this blockage is normally maintained until the SRP binds the ribosome to the ER membrane. Without this blockage, a complete preprolactin preprolactin chain might be produced by cells and released into the cytosol rather than being transported into the lumen.

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You perform a second experiment, in which you add SRP to your cell-free protein-

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