ER Protein Translocation and Membrane Integration
Protein Translocation and Membrane Integration in the ER
This section details the mechanism by which proteins enter the Endoplasmic Reticulum (ER) and integrate into its membrane, focusing on the roles of hydrophobic segments, chaperones, and the translocation machinery.
Role of Hydrophobic Segments and BIP
A hydrophobic segment, specifically a signal sequence, is crucial for membrane integration. Due to its hydrophobic nature, it tends to remain embedded within the membrane.
The process involves the ribosome, mRNA, and the nascent polypeptide chain being synthesized.
The chaperone protein BIP plays a vital role by binding to Sec61, part of the translocon complex.
BIP specifically targets and binds to hydrophobic segments of the polypeptide chain.
The primary reason BIP binds hydrophobic segments is to prevent their aggregation, which commonly occurs with exposed hydrophobic regions.
By binding these segments, BIP facilitates proper protein folding, ensuring the protein attains its correct three-dimensional structure within the ER lumen or membrane.
Mechanisms of Translocation and Membrane Integration
The translocon (often involving Sec61) can exist in a closed confirmation (with a plug) and an open confirmation to allow polypeptide passage.
Translocation can occur sideways into the membrane, not just directly through the channel into the lumen.
Hydrophobic alpha helices are key structures that can form transmembrane domains (TMDs).
These hydrophobic alpha helices, once synthesized, can be laterally displaced from the translocon directly into the lipid bilayer of the ER membrane.
This mechanism is exemplified by certain peptides, such as a sigmoid peptide, which can integrate into the membrane after this lateral displacement.
Types of Membrane Proteins and Orientation
Proteins can be either multi-spanning or single-spanning membrane proteins.
Both types integrate into the membrane via alpha helices rich in hydrophobic amino acids.
These hydrophobic alpha helices are designed to span the lipid bilayer, acting as anchors or functional domains within the membrane.
An example of membrane protein orientation is a protein with its N-terminus located in the ER lumen and its C-terminus in the cytosol.
Rules for Membrane Protein Orientation
Specific rules govern how a sequence emerging from the ribosome determines its path into the ER and its final membrane orientation.
Upon initial synthesis, the polypeptide chain undergoes a kind of "first rotation" or interaction that dictates its subsequent integration.
When visualizing membrane integration pathways, different segments (e.g., colored red and orange) represent membrane-spanning regions.
The distinction between these colored segments is crucial as it helps determine the protein's orientation within the membrane.
Generally, the N-terminal domain of a newly synthesized protein will be found in the cytosol.
However, this orientation rule has an important exception: if the N-terminal domain possesses a signal recognition sequence at its very beginning, it will be directed across the membrane, ending up in the ER lumen or embedded differently.