Protein Targeting to the Endoplasmic Reticulum PDF

Protein Targeting to the Endoplasmic Reticulum 🌐 Overview of the Endoplasmic Reticulum The Endoplasmic Reticulum (ER) is a type of organelle found in eukaryotic cells. It is a network of membranous tubules and cisternae that is spread throughout the cytosol. "The ER is often described as lookin...

Protein Targeting to the Endoplasmic Reticulum 🌐 Overview of the Endoplasmic Reticulum The Endoplasmic Reticulum (ER) is a type of organelle found in eukaryotic cells. It is a network of membranous tubules and cisternae that is spread throughout the cytosol. "The ER is often described as looking like lace, because it's just this kind of lacy network that's spread all throughout the nucleus." Types of Endoplasmic Reticulum There are two types of ER: Rough ER: studded with ribosomes, which are responsible for protein synthesis. The rough ER appears as stacks of pancakes. Smooth ER: lacks ribosomes and is more vesicular in nature. Protein Synthesis and Targeting Most proteins are made in the cytosol, except for those made in the mitochondria. The ribosomes translate the mRNA, and the protein must be targeted to its destination. Cotranslational vs. Post-Translational Events Cotranslational event: the protein is translocated into the organelle as the ribosome is translating the mRNA. Post-translational event: the protein is completely made and then translocated to its destination. Organelle Type of Event Nucleus Post-translational Mitochondria Post-translational (unfolded) Peroxisome Post-translational (folded) Protein Translocation to the ER Protein translocation to the ER is a post-translational event. The protein is completely made and then translocated to the ER. The Secretory Pathway The ER plays a crucial role in the secretory pathway, which is the process by which proteins are transported from the ER to other organelles, such as the Golgi apparatus, lysosomes, and plasma membrane. "The ER is the beginning of the secretory pathway, and we're gonna move proteins in the secretory pathway by using vesicles." Key Points The ER is the starting point for the secretory pathway. 🚂 Protein Proteins must first go to the ER before being transported to other organelles. 🚂 The ER is responsible for protein translocation, folding, and modification.## Transport to the ER The ER (Endoplasmic Reticulum) is a crucial organelle responsible for protein synthesis, folding, and transport. All proteins destined for the secretory pathway, including those that will be secreted outside the cell, embedded in the plasma membrane, or sent to lysosomes, must first pass through the ER. The Importance of the ER The ER is the starting point for protein transport to various destinations within the cell. It's like a bus station where all proteins board the same bus (the ER) before getting off at their final destination. Types of ER There are two types of ER: Type of ER Function Smooth ER Lipid synthesis, including phospholipid synthesis Rough ER Protein synthesis for proteins entering the secretory pathway Structure of the ER The ER is an extension of the outer nuclear membrane and is characterized by its flat, spread-out structure. It's composed of tubules and sheets that are in close proximity to all regions of the cytosol. Protein Targeting to the ER Proteins destined for the ER lumen or membrane must have a signal sequence. The signal sequence is a region of amino acids that directs the protein to the ER. Characteristics of the Signal Sequence Located on the amino terminus of the protein Can be clipped off after protein targeting Contains a region of nonpolar amino acids that are important for protein targeting "The signal sequence is like a ticket that allows the protein to board the bus (the ER) and get to its final destination." Protein Transport into the ER Proteins with a signal sequence are transported into the ER lumen, where they may be modified, folded, or sent to other destinations within the cell. Misfolded Proteins Proteins that are misfolded in the ER can be degraded through a process involving chaperones and the proteasome. In some cases, misfolded proteins can be transported back to the cytosol for degradation. Key Points The ER is the starting point for protein transport to various destinations within the cell. The signal sequence is a crucial region of amino acids that directs proteins to the ER. The ER is composed of smooth and rough ER, each with distinct functions. 📍 Proteins can be transported back to the cytosol for degradation if they are misfolded in the ER.## Signal Sequences and Protein Targeting to the ER Signal Sequence Structure A signal sequence is a 16-13 amino acid sequence located at the amino terminus of a protein. It typically has 1 or more positively charged amino acids adjacent to a hydrophobic core, which is 6-12 amino acids long. The hydrophobic core is the binding site for interaction with the transport receptor. Signal Sequence Function The signal sequence is cleaved from the protein and is not part of the mature protein. Its function is not related to the function of the individual protein, but it may play a role in signaling to the ER that a protein is being imported. Microsomes Microsomes are vesicular structures that form when the ER is isolated and broken apart. They are not a natural component of cells, but rather an artifact of ER isolation. Microsomes can have ribosomes associated with them and are used to study ER function. Protein Translocation to the ER Experiment Conditions Result 1 Cytosol + mRNA Protein is made and fully folded in the cytosol 2 Cytosol + mRNA + rough microsomes Protein is made and translocated into the lumen of the microsome Cotranslational Translocation Cotranslational translocation is the process by which a protein is translocated into the ER lumen at the same time it is being translated. This process is dependent on the presence of the signal sequence and the rough microsomes. Cotranslational translocation is the main way that proteins get to the ER lumen. Why Cotranslational Translocation? Cotranslational translocation is thought to be used because it allows for efficient use of space and energy. The ribosome is not embedded in the ER, but rather docks onto the ER membrane to facilitate protein translocation. Energy Requirements for Protein Translocation Location Energy Source Mitochondria HSP 70 (ATP hydrolysis) and electron transport chain (proton motor force) ER Unknown, but thought to be related to energy requirements for protein folding and translocation Key Points Signal sequences are 16-13 amino acid sequences located at the amino terminus of proteins Signal sequences are cleaved from the protein and are not part of the mature protein Microsomes are vesicular structures that form when the ER is isolated and broken apart Cotranslational translocation is the main way that proteins get to the ER lumen 🌐 Energy requirements for protein translocation to the ER are unknown, but thought to be related to protein folding and translocation.## Membrane Electric Potentials and Proton Motor Force The proton motor force is a specific type of membrane electric potential found at the inner mitochondrial membrane. This potential is created by the difference in charge across the membrane, with the inside of the mitochondria having a negative charge. 🌈 Signal Sequences and Mitochondrial Import Signal sequences are amphipathic alpha helices with a core of hydrophobic amino acids. They have a positive charge due to the presence of lysines and arginines. The signal sequence is 16-30 amino acids long and is located at the amino terminus of the protein. "A signal sequence is a specific sequence of amino acids that directs a protein to a specific location within the cell." 📦 Signal Recognition Particle (SRP) SRP is a complex made of RNA and proteins. It is responsible for recognizing and binding to signal sequences. SRP is composed of: 7sRNA (made by RNA polymerase 3) 🔄 Proteins attached to the RNA SRP Structure and Function SRP has a signal sequence binding pocket that binds to the core of hydrophobic amino acids in the signal sequence. SRP has a translational pause domain that competes with elongation factor for binding to the ribosome. The translational pause domain pauses translation by preventing the ribosome from moving 📈 towards the 3' end of the mRNA. Translational Pause Domain The translational pause domain is a region of SRP that binds to the ribosome and prevents translation from continuing. 🌉 It is necessary to pause translation to allow the ribosome to dock onto the ER membrane. ER Membrane and Translocon The ER membrane has a receptor for SRP that binds to SRP and allows the ribosome to dock. The translocon is a small core embedded in the ER membrane that allows the protein to pass through. The protein is "threaded" through the translocon as it is being made, using the energy of ⚡️ translation to drive the process. Energy of Translation The energy of translation is the energy expended to translate the mRNA into a protein. This energy is used to push the protein through the translocon and into the lumen of the ER. The energy of translation is necessary to drive the process of protein translocation into the ER. Component Function Signal Sequence Directs protein to specific location within cell SRP Recognizes and binds to signal sequence Translocon Allows protein to pass through ER membrane Ribosome Translates mRNA into protein ER Membrane Provides receptor for SRP and translocon for protein translocation The Role of Microsomes Microsomes are essential for protein synthesis, as they provide a site for translation to occur. Without microsomes, the ribosome has nowhere to go, and translation cannot take place. Comparison to Mitochondria The process of protein synthesis in the endoplasmic reticulum (ER) is similar to that in mitochondria in some ways, but also has some key differences. Similarities: The protein moves into the ER in an unfolded state, just like in mitochondria. The receptor is embedded in the membrane, similar to the Tim complex in mitochondria. Differences: The process is cotranslational, meaning it occurs simultaneously with translation, whereas in mitochondria it is post-translational. The ER has only one membrane, whereas mitochondria have two. The Energy of Translation The energy of translation is a unique source of energy that drives protein synthesis in the ER. This energy is generated by the movement of ribosomes along the mRNA. Ribosome Movement and Protein Synthesis Ribosome Movement: The ribosome moves along the mRNA, translating the genetic code into a protein sequence. Protein Synthesis: The protein is synthesized as the ribosome moves along the mRNA, with the signal sequence directing it to the ER. The Role of the Signal Sequence Signal Sequence: A sequence of amino acids that directs the protein to the ER. Signal Peptidase: An enzyme that cleaves the signal sequence from the protein as it enters the ER. The Translocon Definition: A protein complex that forms a channel through which the protein is translocated into the ER. Sec61: A specific type of translocon that is involved in protein secretion. Component Function Ribosome Translates mRNA into protein sequence Signal Sequence Directs protein to ER Signal Peptidase Cleaves signal sequence from protein Translocon Forms channel for protein translocation Sec61 Specific type of translocon involved in protein secretion The Movement of mRNA through the Ribosome mRNA Movement: The mRNA moves through the ribosome, with the ribosome remaining stationary. Protein Synthesis: The protein is synthesized as the mRNA moves through the ribosome. The Role of SRP SRP: A protein complex that binds to the signal sequence and directs the ribosome to the ER. SRP Receptor: A protein that binds to SRP and helps to position the ribosome on the translocon. The History of Sec61 Discovery: Sec61 was discovered through a clever approach involving the creation of a library 🚪 of yeast mutants that failed to secrete proteins. Function: Sec61 is involved in protein secretion and is a key component of the translocon.## Translocon and Protein Translocation The translocon is a complex that forms a channel in the endoplasmic reticulum (ER) membrane, allowing proteins to be translocated into the ER lumen or embedded in the ER membrane. Plug and Lateral Gate The translocon has a plug that blocks the channel when not in use. When a ribosome docks on the translocon, the plug moves out of the way, allowing the protein to pass through. The translocon also has a lateral gate that can open to allow single peptides to migrate laterally out of the gate and become embedded in the membrane. Signal Peptidase Signal peptidase is an enzyme that cleaves the signal sequence from a protein after it has been translocated into the ER lumen. The mitochondria and ER have signal peptidase, but the nucleus does not. The peroxisome does not have a known signal peptidase. Isoleucines as a Gasket The inside of the translocon core is lined with isoleucines, which form a tight gasket around the protein as it passes through. 🔄 This prevents other molecules from passing through the channel. Post-Translational Translocation Post-translational translocation is a mechanism for translocating proteins into the ER after they have been fully synthesized. This process involves the Sec61 complex, as well as other proteins such as Sec62, Sec63, and 🔄 BIP (a chaperone). Protein Folding and Modification After a protein is translocated into the ER lumen, it must fold into its native conformation. Folding is a spontaneous process, but chaperones can assist in the process. Modifications, such as glycosylation, can also occur in the ER.