Describe the passage of a protein molecule from its origin on a prtion of the rough endoplasmic reticulum to the secretion out of the cell.
http://en.wikipedia.org/wiki/Endoplasmic_reticulum
To describe the passage of a protein molecule from its origin on a portion of the rough endoplasmic reticulum to secretion out of the cell, we can follow these steps:
1. Synthesis of the protein: The process begins with the synthesis of the protein on the ribosomes that are bound to the rough endoplasmic reticulum (ER). The ribosomes translate the mRNA into a chain of amino acids, forming a polypeptide.
2. Co-translational translocation: As the protein is being synthesized, it is simultaneously translocated into the lumen of the rough ER. During this process, a signal peptide at the beginning of the polypeptide chain guides the protein into the ER membrane. The signal peptide interacts with a signal recognition particle (SRP), which halts translation and directs the ribosome-mRNA complex to the ER membrane.
3. Insertion into the ER membrane: The ribosome, mRNA, and the polypeptide chain bind to a protein complex called the translocon, which is embedded in the ER membrane. The polypeptide then begins to enter the ER lumen while the ribosome continues to synthesize the protein.
4. Proper protein folding and modification: Once inside the ER lumen, the protein molecule undergoes folding, facilitated by chaperone proteins, to attain its native conformation. The ER also aids in the post-translational modifications of the protein, such as glycosylation, which involves adding sugar molecules to the protein chain.
5. Quality control and sorting: The ER has a stringent quality control system that ensures only properly folded proteins proceed further. Any misfolded or aberrant proteins are identified and tagged for degradation. Additionally, proteins meant for secretion or transport to other organelles are sorted and packaged into vesicles, such as transport vesicles or Golgi vesicles.
6. Transport to the Golgi apparatus: The protein-containing vesicles bud off from the ER and are transported to the Golgi apparatus. The Golgi serves as a processing and sorting center, refining the protein's structure, adding further modifications, and directing it to its final destination.
7. Secretion out of the cell: In the Golgi apparatus, the protein is sorted based on its destination. Some proteins are packaged into vesicles for secretion out of the cell. These vesicles fuse with the cell membrane, releasing the protein into the extracellular space through a process called exocytosis.
Overall, the passage of a protein molecule from its origin on a portion of the rough ER to secretion out of the cell involves a complex series of steps that ensure proper synthesis, folding, modification, and transport to the appropriate location within or outside the cell.