Cell and Molecular Biology: Protein Insertion into the ER Membrane

Summary

This video explores the process of transmembrane protein insertion into the ER membrane. It explains the roles of different signal sequences, including terminal and internal signals, and introduces four types of transmembrane proteins—Type 1, Type 2, Type 3, and Type 4. Each type of protein has a unique insertion mechanism that involves co-translational import, with varying interactions between signal sequences, stop transfer sequences, and the translocon. The video also delves into the topology of each protein type, demonstrating how they anchor themselves in the membrane or traverse it multiple times.

Takeaways

• 😀 Transmembrane proteins and water-soluble proteins both have signal sequences that guide them to the ER for insertion.
• 😀 Transmembrane proteins are synthesized co-translationally by ribosomes and imported through the translocon into the ER membrane.
• 😀 Type 1 membrane proteins have a signal sequence at the N-terminus and an internal stop transfer sequence that embeds the protein in the membrane.
• 😀 Type 2 membrane proteins feature an internal signal sequence that functions similarly to a Type 1 protein's signal sequence but without cleavage by signal peptidase.
• 😀 Type 3 membrane proteins share a similar insertion mechanism to Type 2 proteins but have the opposite topology, with the N-terminus in the cytosol and the C-terminus in the ER lumen.
• 😀 Type 4 membrane proteins are multi-pass proteins that can traverse the membrane multiple times, using alternating stop transfer sequences.
• 😀 The translocon serves as the channel through which proteins enter the ER lumen, and signal peptidase cleaves the signal sequence for proper protein orientation.
• 😀 In Type 1 protein insertion, the stop transfer sequence halts the protein from passing completely through the membrane, embedding it in the bilayer.
• 😀 In Type 2 and Type 3 proteins, the internal signal sequence anchors the protein in the membrane without cleavage, affecting the protein's final topology.
• 😀 The stitching analogy is used to explain the protein insertion mechanism, demonstrating how the 'thread' (protein) is pulled through the 'cloth' (membrane) in different configurations for various protein types.

Q & A

What are the two types of proteins that enter the ER?

The two types of proteins that enter the ER are water-soluble proteins and transmembrane proteins. Both types have signal sequences that direct them to the ER.

What is the function of the signal sequence in transmembrane proteins?

The signal sequence functions as an address label, directing the growing protein to the ER membrane.

What are the characteristics of Type 1 membrane proteins?

Type 1 membrane proteins have a signal sequence at their N-terminus and an internal stop-transfer sequence that helps anchor the protein in the ER membrane.

How does the process of co-translational import occur?

During co-translational import, ribosomes continue synthesizing the protein as it crosses the ER membrane via the translocon, which acts as a channel through the membrane.

What happens to the signal sequence in Type 1 membrane protein insertion?

The signal sequence is cleaved by a membrane-bound signal peptidase after the protein enters the translocon, allowing the stop-transfer sequence to exit laterally into the membrane.

What distinguishes Type 2 membrane proteins from Type 1 membrane proteins?

Type 2 membrane proteins have an internal signal sequence, rather than a terminal signal sequence. The internal signal sequence binds to the translocon and anchors the protein in the membrane without cleavage by signal peptidase.

How are Type 2 membrane proteins inserted into the membrane?

Type 2 proteins are inserted such that their N-terminus remains in the cytosol, while their C-terminus is oriented toward the ER lumen. This occurs without cleavage of the internal signal sequence.

What is the key difference between Type 2 and Type 3 membrane proteins?

Type 3 membrane proteins also have an internal signal sequence, but their topology is the opposite of Type 2 proteins. Their C-terminus remains in the cytosol, and the N-terminus faces the ER lumen.

What is the analogy used to explain the insertion of Type 2 and Type 3 proteins?

The analogy of stitching a needle through a cloth is used to describe how proteins pass through the membrane, with Type 2 making a full stitch and Type 3 making a single stitch.

How do Type 4 membrane proteins differ from Type 1, Type 2, and Type 3 proteins?

Type 4 proteins are multi-pass membrane proteins, which means they have multiple hydrophobic regions that traverse the membrane. Each pass through the membrane is separated by stop-transfer sequences, which halt protein transfer through the translocon.