Name: how multipass proteins enter the er membrane
Uploaded: 2023-04-30T10:07:23.000Z
Duration: 1 min 29 s
Channel: JoVE

Multipass transmembrane proteins have two or more hydrophobic domains embedded across the ER membrane.
Consider a protein with three transmembrane domains. Its N-terminal ER signal sequence acts as the start-transfer cue for polypeptide translocation down the Sec61 channel. This signal sequence is later cleaved by the signal peptidase complex on the ER membrane.
Upon encountering a hydrophobic region, the translocon stops the polypeptide transfer and opens the lateral gate to transfer this domain into the lipid bilayer.
Then, the translation resumes, synthesizing a cytosolic domain until the next transmembrane domain is encountered.
Translocation pauses again for the membrane integration of this second transmembrane domain.
The above cycle repeats to incorporate the third transmembrane domain in the membrane.
After translation terminates, the resultant multipass protein has three transmembrane and four extra-membrane domains with its N-terminal in the ER lumen and the C-terminal in the cytosol.
Proteins with odd-numbered transmembrane domains orient their N and C termini on the opposite sides of the membrane, while proteins with even-numbered transmembrane domains align their termi

insertion of multi-pass transmembrane proteins in the rer

Insertion of Multi-pass Transmembrane Proteins in the RER

The rough ER membrane synthesizes, assembles, and embeds transmembrane proteins in diverse topologies. These proteins function as transporters or channels and can remain in the ER membrane or are sent to the Golgi complex, lysosome, and cell membrane.
The multipass transmembrane proteins are the type IV integral membrane proteins with multiple topogenic sequences determining their spatial arrangement in the ER membrane. Nearly all multipass proteins lack a cleavable signal sequence and use their first hydrophobic or transmembrane domain as the ER signal sequence. The positioning of positive residues before or after the first transmembrane domain determines the orientation of the N terminal of the protein in the cytosol or lumen, respectively.
The signal recognition particle (SRP) and its receptor (SR) are required to initiate the translocation of the first transmembrane domain of a multipass membrane protein through the ER membrane. The threading of subsequent transmembrane domains is independent of the SRP-SR complex. It is managed by the ribosome-translocon assembly and is primarily dependent on the hydrophobicity of the translated domain. The N-terminal of multipass proteins like glucose transporters and Sec61 lies in the cytosol, while that of G protein-coupled receptors are placed in the ER lumen.
The insertion of single-pass transmembrane proteins is understood better as compared to the multipass transmembrane proteins. The multipass proteins have complex biophysical and topological features affected by the length and hydrophobic profile of their transmembrane domains, the distance between consecutive transmembrane domains, and the length of the extra-membrane loops. The Sec61 translocon-ribosome assembly forms the core of protein translocation of membrane proteins. However, it is an overly simplified model to explain the incorporation of multipass proteins. Different accessory complexes like TRAP and TRAM interact with the Sec61 translocon and assist in accurate folding, insertion, and assembly of multipass transmembrane proteins.

The rough ER membrane synthesizes, assembles, and embeds transmembrane proteins in diverse topologies. These proteins function as transporters or channels ...

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Education

JoVE Core

Cell Biology

Unit 1

Transmembrane Transport in Endoplasmic Reticulum and Peroxisomes

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KEY TERMS AND CONCEPTS

how multipass proteins enter the er membrane

Multipass transmembrane proteins enter the rough ER membrane through a stepwise insertion process. These proteins are type IV integral membrane proteins, and they use multiple topogenic sequences to set their final position in the membrane. They can stay in the ER or later move to the Golgi complex, lysosome, or cell membrane.

Most multipass proteins do not have a cleavable signal sequence. Instead, their first hydrophobic segment, also called a transmembrane domain, acts as the ER signal sequence. The placement of positive residues before or after this first domain helps decide whether the N terminus faces the cytosol or the ER lumen.

The signal recognition particle, or SRP, and its receptor, SR, start the translocation of the first transmembrane domain across the ER membrane. After that first step, later transmembrane domains are threaded in a way that does not depend on the SRP-SR complex. Their insertion depends mainly on the ribosome-translocon assembly and on the hydrophobicity of the translated segment.

Examples help show the range of membrane orientation. In glucose transporters and Sec61, the N terminus lies in the cytosol. In G protein-coupled receptors, the N terminus is placed in the ER lumen. Compared with single-pass proteins, multipass proteins have more complex biophysical and topological features, which are shaped by transmembrane domain length, hydrophobic profile, the spacing between domains, and the length of the loops outside the membrane.

The Sec61 translocon-ribosome assembly is the core machine for membrane protein translocation. However, it does not explain multipass protein insertion on its own. Accessory complexes such as TRAP and TRAM work with Sec61 to help the protein fold, insert, and assemble correctly.

How Multipass Proteins Enter the ER Membrane

how-multipass-proteins-enter-the-er-membrane

the endoplasmic reticulum

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The endoplasmic reticulum, or ER, is a major membrane network in eukaryotic cells. It makes up more than half of a cell’s membranes and about 10% of total cell volume. The ER also serves as a main site for protein and lipid synthesis for organelles such as the Golgi apparatus, lysosomes, secretory vesicles, and the plasma membrane. The ER was also the last major cell organelle to be discovered. In 1954, Keith Porter and George Palade produced the first high-resolution electron microscope...

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protein translocation machinery on the er membrane

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The translocon complex on the ER membrane acts as the main gateway for the protein secretory pathway. It moves new peptides into the ER lumen and also helps insert proteins into the ER membrane. In eukaryotes, the core of this complex is the Sec61 channel. Sec61 is a heterotrimer made of Sec61α, Sec61β, and Sec61γ, and it is the largest part of the translocon. In higher eukaryotes such as mammals, the channel is capped when it is resting. This helps prevent ions such as calcium from leaking...

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insertion-of-single-pass-transmembrane-proteins-in-the-rer

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Single-pass membrane protein topology in the ER explains how certain proteins are built into a cell membrane. Integral membrane proteins are attached to the lipid bilayer of a cell organelle or membrane. They may span the membrane as transmembrane proteins, or they may sit on one side only as monotopic proteins. Transmembrane proteins have both membrane-spanning and extra-membrane domains. Their transmembrane regions usually contain 20-25 hydrophobic amino acids arranged in a helical shape.

Multipass transmembrane proteins enter the rough ER membrane through a stepwise insertion process. These proteins are type IV integral membrane proteins, and they use multiple topogenic sequences to set their final position in the membrane. They can stay in the ER or later move to the Golgi complex, lysosome, or cell membrane. Most multipass proteins do not have a cleavable signal sequence. Instead, their first hydrophobic segment, also called a transmembrane domain, acts as the ER signal...

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synthesis-of-phosphatidylcholine-in-the-er-membrane

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Phosphatidylcholine is a major glycerophospholipid in cell membranes. The endoplasmic reticulum, or ER, makes lipids that help build membranes and support cell functions such as energy storage and signaling. The lipid-making machinery sits in the ER membrane and gathers its reactants from the cytosol, the fluid inside the cell. After phosphatidylcholine and other lipids are made, the cell moves them to other organelles through the secretory pathway and through ER contact sites. Energy-rich...

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peroxisomes

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key terms and concepts

Learn how multipass transmembrane proteins use SRP, Sec61, TRAP, and TRAM to insert into the rough ER membrane and set their final orientation.

How Multipass Proteins Enter the ER Membrane | JoVE K12

cell biology

JoVE Core Cell Biology illustrates complex cellular processes in easy-to-understand yet comprehensive animated video lessons. This collection focuses on the essential components and functions of cells, including organelles and membranes, cellular transport, and cell energetics. It is complemented by scientist-in-action videos that demonstrate the application of cell biology in real-world research experiments.

How Multipass Proteins Enter the ER Membrane

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