Polypeptide starts on a free ribosome. In the first step, the signal sequence emerges from the ribosome and binds to the SRP, which stops further translocation until the SRP-ribosome-nascent chain complex can make contact with the ER membrane. The SRP-ribosome then binds to an SRP receptor within the ER membrane during step 2. In the third step SRP is released and the association of the ribosome with a translocon of the ER membrane occurs. These latter events are accompanied by the reciprocal hydrolysis of GTP molecules bound to SRP & its receptor. In the fourth step, the signal peptide then binds to the inside of the translocon, displacing the plug from the channel. This allows the polypeptide to translocate through the membrane without constraint. After this polypeptide passes into the lumen of the ER, the signal peptide is cleaved by a membrane protein & the protein undergoes folding with the aid of BiP and other ER chaperones. The Signal Recognition Hypothesis states that secretory proteins have a signal sequence & contain built-in “address codes.” This plays an important role in this synthesis process because this hypothesis applies to nearly all protein trafficking pathways.
Dr. George Palade, with the help of James Jamieson, used a technique called autoradiography to follow a cell cycle from start to finish. Autoradiography allows the finding of radioactive materials within a cell. To determine where secretory proteins are synthesized, Palade incubated slices of pancreatic tissue in radioactive amino acids for a short time period. By doing this, Palade & Jamieson found that the site of synthesis of secretory proteins is the endoplasmic reticulum. To follow the path of secretory proteins, the researchers furthered their experiment. They did this by washing the tissue and transferred it to a medium of unlabeled amino acids. This method is a “pulse-chase.” The pulse is the incubation period and the chase occurs when the tissue is...
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