Protein_sorting_and_targeting___PPT.pptx

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1. PROTEIN SORTING

2. POLYSOME A polysome is a complex of multiple ribosomes that are simultaneously translating a single mRNA molecule into proteins. This structure allows for the rapid and efficient synthesis of many copies of the same protein from one mRNA strand, as each ribosome works on the same genetic code at the same time.

3. Co - translational transport Cotranslational transport is the process by which ribosomes synthesizing proteins with specific signal sequences are guided to the endoplasmic reticulum (ER) and insert the growing polypeptide chain directly into or across the ER membrane as it is being made. This pathway, mediated by the signal recognition particle (SRP) and the ER translocation channel (translocon), is essential for the proper targeting and insertion of most secretory and membrane proteins into the cell's secretory pathway.

4. Key Steps in Cotranslational Transport: Signal Sequence Recognition: As a protein is synthesized, its N-terminal signal sequence emerges from the ribosome and is recognized by the signal recognition particle (SRP). Targeting to the ER: The SRP-ribosome complex, now bound to the nascent polypeptide, is targeted to the ER membrane via the interaction of the SRP with its receptor.

5. Transfer to the Translocon: The complex then docks at the ER membrane, and the nascent chain is transferred from the SRP to the Sec61 protein translocation channel (translocon). Translocation: The polypeptide chain is threaded through the translocon into the ER lumen or inserted into the ER membrane. Completion of Synthesis: The protein continues to be synthesized while it is translocated or integrated into the membrane.

6. DIFFERENCE Cotranslational translocation is the direct insertion of a nascent (newly forming) polypeptide chain into a cellular membrane, like the endoplasmic reticulum (ER), while it is still being synthesized by a ribosome. In contrast, post-translational translocation involves a completed polypeptide that has already been synthesized in the cytosol being transported to its destination, such as the ER, mitochondria, or nucleus, after its translation is finished. The key difference lies in whether the protein moves across the membrane during its synthesis (cotranslational) or after its synthesis is complete (post-translational).

7. Cotranslational Translocation Timing: Occurs simultaneously with protein synthesis. Mechanism: A ribosome synthesizing a protein with a signal sequence is directed to the ER membrane, where the growing polypeptide chain is fed through a protein-conducting channel while still attached to the ribosome. Energy: Requires energy to guide the ribosome to the membrane and to move the polypeptide through the pore. Examples: The pathway for most secreted proteins and proteins destined for the ER, Golgi apparatus, plasma membrane, or other organelles in the secretory pathway.

8. Post-Translational Translocation Timing: Happens after the polypeptide is fully synthesized. Mechanism: The completed protein is released into the cytosol, then recognizes and associates with the protein's destination membrane, moving across a channel. Energy: Requires energy and molecular chaperones to assist in keeping the protein in a translocatable state and to facilitate its passage through the channel. Key components include chaperones, which keep the unfolded protein from misfolding , and the Sec translocon complex, which forms a channel for the protein to pass through. Examples: Proteins targeted to the mitochondria, chloroplasts, peroxisomes. Nucleoproteins targeted to the nucleus via nuclear pores.

9. MITOCHONDRIA

10. Translocation across membranes TOM complex: The protein is passed through the translocase of the outer membrane (TOM complex), which is a channel in the outer mitochondrial membrane. TIM complex: After crossing the TOM complex, the protein is handed off to the translocase of the inner membrane (TIM complex). Proton gradient: Translocation through the TIM complex requires an electrochemical gradient of protons (H+) across the inner mitochondrial membrane to provide the energy needed to pull the protein through the TIM channel

11. Post-translational transport into chloroplasts process of moving proteins, synthesized in the cytosol, into different compartments of the chloroplast after their translation. This is a multi-step process that involves the recognition of an N-terminal signal sequence, binding to the TOC (Translocon at the Outer Envelope Membrane of Chloroplasts) and TIC (Translocon at the Inner Envelope Membrane of Chloroplasts) complexes, and energy-dependent translocation across the outer and inner membranes. The protein is then folded and processed, often with its signal sequence removed

12. PEROXISOME Post-translational transportation into peroxisomes involves the import of fully synthesized, folded proteins from the cytosol. This process relies on specific peroxisomal targeting signals (PTS) on the proteins, which are recognized by receptor proteins in the cytosol. These receptors then escort the protein complexes to a receptor in the peroxisomal membrane, where the proteins are translocated into the matrix before the receptor is recycled.

13. Recognition by receptors: Proteins destined for the peroxisome have a specific targeting signal, known as a peroxisomal targeting signal (PTS). Most proteins use a PTS1 at the C-terminus (e.g., the tripeptide SKL), while a smaller subset uses a PTS2 near the N-terminus. Receptor proteins in the cytosol, like PTS1R , bind to these PTSs.

14. Docking at the peroxisome: The PTS receptor protein complex then docks at the peroxisomal membrane, interacting with docking proteins, such as Pex14. Translocation: The protein complex is translocated across the membrane into the peroxisomal matrix. Release and recycling: Once inside, the protein dissociates from the PTS receptor, which is then recycled back into the cytosol to facilitate the transport of another protein.

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