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Rough Endoplasmic Reticulum

Submitted by rmmcdonald on Fri, 11/08/2019 - 13:54

The main function of the rough ER is to synthesize, process, and transport proteins. This biological process begins in the cytosol where a ribosome initiates the translation of an mRNA strand. The ribosome may translate a short signal sequence on N-terminus that marks the polypeptide as destined for the ER. A signal recognition particle, or SRP, will recognize this sequence and bind to the polypeptide (“Molecular Biology”). Figure 3 reflects the process of translation in two different scenarios: when a polypeptide expresses an ER signal sequence or no signal sequence. With the SRP guiding, the complex is then localized to a translocon on the rough ER membrane. As the ribosome creates a protein strand based off of the mRNA transcript, the channel protein allows the polypeptide chain to enter the ER lumen where protein folding commences. This process where the still synthesizing polypeptide enters the ER is known as co-translation (Cooper). Within the ER lumen are a variety of chaperone proteins that help fold polypeptides. These chaperone proteins contain an ER retention sequence so that they remain within the ER (Cooper). Other proteins that have entered the ER to be processed contain only a temporary ER signal sequence and exhibit another signal sequences for a different cellular location. In that case, proteins will be packaged into a vesicle, a membrane bound sack, that buds off of the ER. This vesicle travels along microtubules, the “backbones” of the cell, until it fuses with the membrane of the Golgi (“Transport from the ER”). The Golgi acts as the final step to process and transport the proteins to their ultimate location in the cell. Figure 4 reflects the general pathway that proteins follow depending on the individual protein’s signal sequence.