ProteinSorting part 2

Summary

This transcript delves into the mechanisms behind protein targeting and translocation within cells. It explains the importance of signal sequences, which direct proteins to specific organelles like the endoplasmic reticulum, nucleus, and mitochondria. The text covers three main transport mechanisms: nuclear pore transport, translocator proteins for organelles, and vesicular transport. It highlights how chaperone proteins such as Hsp70 aid in protein folding and translocation, emphasizing the role of ATP hydrolysis in these processes. The detailed explanation of mitochondrial protein import underscores the complexity and precision of cellular protein management.

Takeaways

• 😀 Proteins are guided by signal sequences to be directed to specific compartments in the cell, such as the endoplasmic reticulum (ER).
• 😀 A signal sequence, often 8-29 amino acids long, plays a crucial role in directing proteins to their intended cellular location.
• 😀 The presence of signal sequences ensures proteins are transported to the correct organelle or compartment, such as the ER or mitochondria.
• 😀 If the signal sequence is removed, proteins may fail to reach their target compartment and could end up in the cytosol instead.
• 😀 There are three main ways proteins are transported within cells: via nuclear pores, through protein translocators in organelle membranes, and via vesicular transport.
• 😀 The nuclear pore complex controls the entry of large molecules into the nucleus and is selective about which molecules pass through.
• 😀 Proteins that enter the nucleus must bind with nuclear transport receptors and pass through the nuclear pore complex.
• 😀 Proteins destined for mitochondria or other organelles must pass through protein translocators embedded in the organelle membranes.
• 😀 Proteins entering mitochondria must remain unfolded during transport, facilitated by chaperone proteins like Hsp70, which help maintain the protein’s linear structure.
• 😀 In the mitochondria, the signal sequence of a protein is cleaved once it reaches the matrix, allowing the protein to fold into its functional three-dimensional structure.

Q & A

What is the significance of signal sequences in protein targeting?

Signal sequences are crucial for directing proteins to the correct compartment within the cell. They determine where the protein should be localized, such as the endoplasmic reticulum or the nucleus.

How do signal sequences differ for proteins destined for the endoplasmic reticulum and those for other organelles?

Proteins destined for the endoplasmic reticulum have a signal sequence consisting of 10 amino acids, whereas those for other organelles, such as the nucleus, typically have an 8-amino acid signal sequence.

What happens if a signal sequence is removed from a protein?

If the signal sequence is removed, the protein may fail to reach its intended destination. For example, a protein that should be in the endoplasmic reticulum might remain in the cytosol instead.

What are the three mechanisms for protein transport between cellular compartments?

The three mechanisms are: 1) Transport through nuclear pores, 2) Transport via protein translocators in organelle membranes, and 3) Transport using vesicles for proteins moving from the endoplasmic reticulum.

How do proteins enter the nucleus from the cytoplasm?

Proteins enter the nucleus through nuclear pores, which are complexes formed by proteins. These pores allow small molecules to pass but are selective for larger molecules, which require transport receptors to enter.

What role do nuclear transport receptors play in protein entry into the nucleus?

Nuclear transport receptors bind to proteins, guiding them to the nuclear pore. Once the complex reaches the pore, the receptor helps the protein pass through by widening the pore for entry.

How does protein transport occur into mitochondria?

Proteins targeting mitochondria are recognized by receptors on the outer membrane. They are then transported via protein translocators, passing through both the outer and inner membranes of the mitochondria.

What is the function of HSP70 in protein transport to mitochondria?

HSP70 is a chaperone protein that helps unfold and straighten the protein during transport to the mitochondria. It ensures the protein remains unfolded so it can pass through the translocators.

What happens to the signal sequence once the protein reaches the mitochondria?

Once the protein enters the mitochondria, the signal sequence is typically cleaved by specific enzymes, allowing the protein to fold properly and function within the matrix.

What are protein translocators, and how do they function?

Protein translocators are membrane-bound proteins that facilitate the movement of other proteins across cellular membranes. In organelles like mitochondria, these translocators help proteins enter and sometimes require additional chaperone proteins like HSP70 for efficient transport.