ER Luminal Proteins and Signal Recognition

ER Protein Synthesis and Modification

• ER luminal proteins have a hydrophobic signal sequence (more than 8 contiguous hydrophobic residues) at the N-terminal, while integral membrane proteins have this sequence internally.
• The signal sequence is recognized by the signal recognition particle (SRP) as soon as it is translated, leading to translation arrest.
• The SRP-nascent peptide chain associates with the SRP receptor on the ER membrane, and then with a translocon (ER membrane channel) to check the signal sequence.
• The nascent peptide is inserted into the translocon in the appropriate orientation, which is crucial as it cannot be altered later.

Protein Modification in the ER Lumen

• As the growing peptide emerges into the ER lumen, it is scrutinized by enzymes such as protease, which cleaves any N-terminal signal sequence.
• Oligosaccharyl transferase identifies asparagine residues and adds pre-assembled sugar residues in a process known as N-linked glycosylation.
• Chaperone proteins, such as BiP, help the growing peptide fold into its correct shape.

ER Quality Control and Degradation

• Misfolded ER proteins are removed through the ER-associated degradation (ERAD) pathway, which involves ubiquitin ligase and the attachment of ubiquitin to lysine residues.
• More than 30% of all ER proteins are marked for degradation.
• Defects in the ERAD pathway can lead to diseases, such as cystic fibrosis and Parkinson's disease.

Protein Trafficking from the ER

• Proteins are packed into vesicles and transported to other organelles, with identity labels such as Rab and lipids.
• Rab is a small G-protein that is active when bound to GTP and inactive when bound to GDP, and plays a crucial role in organelle recognition.
• Defective Rabs can cause diseases, such as Charcot-Marie-Tooth disease and legionnaire's disease.

Golgi Apparatus and Protein Modification

• Proteins folded and N-glycosylated in the ER are collected into COPII vesicles to move to the cis Golgi.
• Within the Golgi, sugar structures of most proteins are trimmed and modified, and proteins directed for lysosomes are marked with mannose-6-phosphate (M6P).

Lysosomal Function and Disease

• Soluble proteins destined for lysosomes are recognized by the M6P receptor, which binds to the M6P modification.
• Many enzymes are uniquely expressed in lysosomes, and genetic differences in these enzymes can lead to lysosomal storage diseases (LSDs).
• Enzyme replacement therapy (ERT) is a treatment for LSDs, which involves administering M6P-modified forms of missing enzymes.

Endocytosis and LDL Receptor

• Low density lipoprotein (LDL) receptors bind to the ApoB-100 component of LDL and are internalized through clathrin-coated pits.
• In the acidic lumen of endosomes, LDL dissociates from its receptor, and is then passed to lysosomes for degradation.
• The LDL receptor is recycled to the plasma membrane, making up to 100 trips before being degraded.