Proteins are manufactured from the "blueprints" found on DNA. After they are translated, they are moved through a system of internal membranes before being distributed throughout the rest of the cell. At some point in this process, they are modified to their functional form.
• Describe the movement of proteins from their manufacturing point through this internal membrane system.
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The movement of proteins from their manufacturing point through the internal membrane system involves the following steps:
1. Protein synthesis: Proteins are initially synthesized on ribosomes in the cytoplasm or on the rough endoplasmic reticulum (ER), depending on the type of protein being produced.
2. Rough endoplasmic reticulum (ER): Many proteins undergo modifications and folding as they are being synthesized on the rough ER. As the ribosomes translate the mRNA, the growing polypeptide chains are directed into the ER lumen or ER membrane.
3. Signal sequence recognition: Some proteins have specific signal sequences, known as signal peptides, which tag them for transport into the ER. These signal peptides are recognized by signal recognition particles (SRP) that bind to the ribosomes and direct them to the ER membrane.
4. ER membrane translocation: Once the ribosome associated with the growing polypeptide chain reaches the ER membrane, the signal peptide interacts with a protein complex called the translocon. The translocon forms a channel through the ER membrane, enabling the nascent protein to be translocated across the membrane into the ER lumen.
5. Protein folding and modifications: In the ER lumen, chaperone proteins help ensure proper folding of the nascent protein. Furthermore, certain modifications may occur, such as the formation of disulfide bonds between cysteine residues, glycosylation, or addition of other functional groups. These modifications often contribute to the protein's stability or activity.
6. Transport to the Golgi apparatus: Once the protein has attained its functional form in the ER, it is packaged into transport vesicles called COPII vesicles. These vesicles bud off from the ER and transport the proteins to the Golgi apparatus.
7. Golgi apparatus: Within the Golgi apparatus, further modifications and sorting of proteins take place. The Golgi stacks consist of distinct compartments called cisternae, which modify and package proteins into vesicles for specific destinations.
8. Vesicle trafficking and secretion: Vesicles containing modified proteins bud off from the Golgi apparatus and move towards the plasma membrane or other organelles. They fuse with their target membrane, delivering the proteins to their specific destinations within the cell or for secretion outside the cell.
Overall, the movement of proteins through the internal membrane system involves a complex and highly regulated process to ensure that proteins are correctly folded, modified, and transported to their appropriate cellular compartments.