Protein Sorting part 3

Summary

This video script explains the complex process of protein targeting and translocation within cells, focusing on mitochondria, chloroplasts, and the endoplasmic reticulum. It discusses the various signal sequences and translocators involved in moving proteins to specific locations within these organelles. The script outlines the mechanisms by which proteins are inserted into membranes, translocated into lumens, and how signal peptides are cleaved. Different pathways for protein transport and insertion into organelle membranes, such as mitochondria and chloroplasts, are also explored, emphasizing the importance of specific signal sequences in guiding these proteins to their final destinations.

Takeaways

• 😀 The mitochondria have distinct regions where proteins can be localized: the matrix, inner membrane, and intermembrane space.
• 😀 Proteins targeting the inner mitochondrial membrane require two signal sequences: a signal sequence and a second signal sequence that is cleaved by peptidases.
• 😀 The OXA complex is involved in inserting proteins into the inner mitochondrial membrane, allowing proteins to integrate into the membrane.
• 😀 Some proteins are synthesized directly by the mitochondria, utilizing its own ribosomes, and follow a similar import mechanism to proteins imported via the OXA complex.
• 😀 For proteins targeting the intermembrane space of mitochondria, signal sequences on the protein ensure correct positioning and insertion.
• 😀 Chloroplasts, unlike mitochondria, have a more complex membrane structure, with three membranes: outer, inner, and thylakoid membranes.
• 😀 Proteins destined for the thylakoid space of chloroplasts require both a signal sequence and a thylakoid-specific signal sequence.
• 😀 The protein translocator on the outer membrane of the chloroplast helps in the recognition and import of proteins into the stroma, where their signal sequences are cleaved.
• 😀 The proteins that enter the thylakoid membrane may either use a spontaneous pathway or a pathway involving signal recognition particles, similar to the mitochondria import mechanism.
• 😀 The endoplasmic reticulum (ER) can either house free ribosomes or ribosomes attached to its membrane. These ribosomes synthesize proteins that are either secreted or integrated into the ER membrane.
• 😀 The Signal Recognition Particle (SRP) guides ribosome-protein complexes to the ER membrane, where proteins are translocated into the ER lumen or integrated into the ER membrane based on their signal sequences.

Q & A

What are the possible destinations for proteins entering the mitochondria?

Proteins entering the mitochondria can be directed to the matrix, the inner mitochondrial membrane, or the intermembrane space between the inner and outer mitochondrial membranes.

What sequence signals are required for a protein to be inserted into the inner mitochondrial membrane?

A protein destined for the inner mitochondrial membrane requires two sequence signals: a signal sequence and a second signal sequence that appears in the matrix. The second signal sequence is cleaved by peptidase to facilitate the protein's insertion into the membrane.

How do proteins synthesized by mitochondria's own ribosomes integrate into the inner membrane?

Proteins synthesized by mitochondria's own ribosomes can integrate into the inner mitochondrial membrane using the same Oxa complex as externally imported proteins.

What role do the 'stop transfer' sequences play in the insertion of proteins into the mitochondrial membrane?

'Stop transfer' sequences are involved when a protein is targeted to the intermembrane space of the mitochondria. These sequences prevent further insertion into the inner membrane and allow the protein to remain in the intermembrane space.

What is the structure of chloroplast membranes, and how does it impact protein targeting?

Chloroplasts have three membranes: an outer membrane, an inner membrane, and a thylakoid membrane. Proteins targeted to the thylakoid space require specific signal sequences, including a plastid targeting signal and a thylakoid signal sequence.

How do proteins enter the stroma of the chloroplast?

Proteins enter the chloroplast stroma through a translocator in the outer membrane. Once in the stroma, the chloroplast signal sequence is cleaved, and the thylakoid signal sequence helps the protein pass through the thylakoid membrane.

What is the role of the Signal Recognition Particle (SRP) in the synthesis and targeting of proteins to the endoplasmic reticulum?

The SRP guides ribosomes synthesizing proteins destined for the endoplasmic reticulum (ER) membrane. It binds to the ribosome, directing the ribosome to the ER membrane where the protein is inserted into the ER lumen.

What happens when a protein is targeted to the lumen of the endoplasmic reticulum?

When a protein is targeted to the lumen of the ER, the SRP helps the ribosome dock on the ER membrane. The protein is synthesized directly into the ER lumen, where it undergoes further processing.

What is the difference between a hydrophobic 'stop transfer' sequence and a hydrophobic 'start transfer' sequence?

A 'start transfer' sequence initiates the insertion of a protein into a membrane, while a 'stop transfer' sequence halts its insertion, anchoring the protein in the membrane. The stop transfer sequence can also cause the protein to span the membrane multiple times.

How does the process of post-translational protein translocation in bacteria differ from eukaryotes?

In bacteria, the protein translocation process is simpler, typically involving fewer proteins like SecA and a translocator to move proteins from the cytosol into the lumen of the ER. Eukaryotes use more complex systems with additional proteins and pathways.