Intracellular Compartments and Protein Sorting PDF

This slide introduces a famous experiment by...

This slide introduces a famous experiment by George Palade, who demonstrated the Microsomes are small vesicles formed from Lateral Diffusion (Within continuous secretory pathway in pancreatic cells: the ER after cell homogenization (breaking cells membranes, ER to Nucleus) apart). They retain the same functionality and Pulse-chase experiment: Radioactively polarity as the ER, which makes them useful for labeled proteins were tracked through the Vesicular Transport (ER to Golgi) **Lipid Transport** cell, revealing the pathway of secretion: studying ER functions in a lab setting. These vesicles helped demonstrate the signal proteins are synthesized in the RER, then Transfer to Mitochondria or Chloroplasts Phospholipid Exchange Proteins hypothesis, which explains how proteins are transported to the Golgi apparatus, targeted to the ER during synthesis. packaged into vesicles, and finally secreted outside the cell. Protein Maturation Trimming of Glycans (Glycosylation modification) **Golgi Apparatus** Anterograde Transport (ER → Golgi → Plasma membrane) Vesicular Sorting: Retrograde Transport (Golgi → ER) Proteins are primarily synthesized by ribosomes, either free in the cytosol or bound to the RER -Proteins destined for the ER typically have a Co-translational Translocation 20-amino acid hydrophobic sequence at Proteins have specific signals or tags, like a peptide signal, that direct them to their their N-terminus, which serves as an proper compartment, whether it's the ER, mitochondria, or nucleus (called the secretory addressing signal to the ER. 1: The signal peptide is recognized by the pathway) Protein Synthesis -The signal can be cleaved off once the protein signal recognition particle (SRP), which binds enters the ER lumen and halts translation. 2:The complex is then directed to the SRP Proteins move across membranes (e.g., into the ER) in a process that can be either co- receptor on the ER membrane. Translocation (Across membranes) translational (during synthesis) or post-translational (after synthesis) 3:The SRP is released, and the ribosome **Protein Sorting and Transport** attaches to the translocon, allowing the peptide to enter the ER. Protein Translocation mechanism: Mindmap for Vesicular Transport This involves the movement of proteins between organelles via There are specific start-transfer and stop- vesicles, which can go from the ER to the Golgi (anterograde) or back 4:Translation resumes, and the protein is threaded into the ER lumen. "Intracellular transfer sequences (short stretches of hydrophobic amino acids) that guide the Peroxisomes 5:In the case of soluble proteins, the signal peptide is usually cleaved by signal peptidase, Compartments and insertion into the membrane, dictating which parts of the protein will remain inside the ER and the protein is fully translocated into the ER. Protein Sorting" Protein Targeting to Organelles: Mitochondria lumen and which parts will face the cytosol have an N-terminal signal peptide and a stop-transfer Nucleus (Transport through nuclear pores) important for proteins destined for the nucleus Type I peptide, which anchors them in the membrane proteins use an internal signal peptide that determines their orientation Type II **Rough Endoplasmic Reticulum (RER)** in the membrane based on the distribution of positively charged residues Transmembrane Proteins Types: Similar to Type II but with positively charged Type III amino acids following the hydrophobic segment. Multipass proteins with combinations of signal Type IV and stop-transfer sequences. Protein Folding and complex formation (Chaperones like BiP) -A 14-sugar oligosaccharide is assembled on the lipid dolichol. -The oligosaccharide is transferred to specific asparagine residues (Asn-X-Ser/Thr) on the N-Glycosylation, oligosaccharide (sugars) are added to the protein. amine grp (N) of the asparagine (side chain) of the protein -Some sugars are trimmed as part of the Once proteins are inside the ER, they undergo maturation process, which continues in the several post-translational modifications Golgi apparatus. Eg: After protein synthesis is complete, transamidase cleaves Some proteins are also modified with glycolipids, such as -A network of interconnected membranes that extend from the protein’s C-terminus and attaches it to a GPI anchor. GPI anchors, which help tether them to membranes the nuclear envelope. -The internal space of the ER is called the lumen, and it Proteins in the ER undergo strict quality control. Misfolded or represents a large portion of the cell's membrane surface area. **Endoplasmic Reticulum (ER)** improperly glycosylated proteins are recognized by the chaperone, Quality Control Mechanisms -The ER can be either rough (RER), with ribosomes attached exported to the cytosol, and degraded by the proteasome. A key ER to its surface, or smooth (SER), which lacks ribosomes. chaperone protein involved in this process is BiP. Fatty acids are delivered to the ER by cytosolic fatty acid binding proteins These fatty acids are linked to glycerol-3-phosphate to form phosphatidic acid, Lipid Synthesis mechanism which serves as a precursor for the synthesis of other phospholipids Phosphatidylcholine is one of the most important phospholipids synthesized in animal cells enzymes help transfer lipids between the two layers of the important for proper membrane function. membrane to equalize the lipid composition across the bilayer phospholipid translocators Flipflops are responsible for actively moving specific phospholipids from one side of the bilayer to the other. This process is ATP-dependent Lipids can move laterally within continuous membrane structures, such as from the ER to Lateral Mobility: the nuclear envelope, because these membranes are physically connected. **Smooth Endoplasmic Reticulum (SER)** Lipids can be transported in vesicles to Lipid Transport to Other Compartments: downstream organelles, such as the Golgi Vesicular Transport how lipids synthesized in the SER are apparatus and beyond. This is a common mechanism in the secretory pathway. transported to other cellular compartments Lipids are transported by phospholipid exchange proteins to organelles like Phospholipid Exchange Proteins mitochondria and chloroplasts, which are not connected to the vesicular transport system Steroid Hormone Synthesis (from Cholesterol) Calcium Storage (Ca²⁺ ATPase in muscle) Hydroxylation of toxins for excretion (e.g., Molecular Detoxification phenobarbital)