Functions of the Endoplasmic Reticulum (ER)

Functions of the Endoplasmic Reticulum (ER)

Overview

• Introduction to ER functions in protein translation and quality control.

• Aim: To ensure proteins are properly folded before being packaged into vesicles for transport.

Structure of the ER

• Network of Membranes: The ER is a complex, interconnected system of membranes visible under a microscope.

  • Appears as a stack of membranes with ribosomes studded on the surface, indicating active protein translation.

  • Compared to the nucleus, the ER is flatter with more membrane area and less content.

Primary Function

• Quality Control for Proteins:

  • Essential role of the ER in verifying the proper folding of proteins before they are dispatched to the Golgi apparatus or other vesicles.

  • Function analogous to a post office checking packages for hazardous materials.

The Process of Quality Control

N-Linked Glycosylation

• Introduction of sugars onto the nitrogen of specific amino acids (particularly asparagines).

  • This process is referred to as N-linked glycosylation.

  • N indicates nitrogen, where sugars are added to specific amino acids.

  • Example of a sugar structure:

  • Contains various types of sugars, including several N-acetylglucosamines, glucose, and mannose.

Role of Recognition Proteins

• Once sugars are bound to the protein, a recognition protein engages with the sugars to check the protein's folding status.

• Calnexin:

  • A chaperone protein embedded in the ER membrane with binding sites for sugars attached to misfolded proteins.

  • Functions as a quality control mechanism, examining proteins for correct folding.

  • Typically localized within the ER to assist in managing protein quality in that organelle.

Checking Mechanism

• Calnexin searches for hydrophobic regions exposed on the protein's surface, indicating improper folding.

  • Normal hydrophobic regions should be buried within the protein's core to avoid contact with water.

  • Incorrect exposure triggers action from calnexin, which cleaves the attached sugars and reverts the protein back in the folding cycle.

Protein Folding Cycle

• Misfolded proteins are repeatedly cycled through interventions by calnexin until properly folded.

• Once correctly folded, proteins are packaged into vesicles for transport away from the ER.

Response to Accumulated Misfolded Proteins

• If an excess of misfolded proteins is detected:

  • They bind to transmembrane proteins located in the ER membrane.

  • This binding activates signaling pathways to manage the cellular response to the overload:

  • Inhibition of Translation: Prevents synthesis of additional proteins to alleviate burden on the ER.

  • Activation of Chaperone Genes: Increases the number of chaperones available to assist in proper protein folding.

Synthesis and Folding of Proteins

• Distinction between synthesis of proteins by free ribosomes in the cytoplasm versus on the ER, with the latter involving both synthesis and folding processes occurring within the ER itself.

• The ER serves as a self-regulatory system to maintain homeostasis, especially essential for managing protein folding and quality control processes.