What is the Difference Between Cytosolic and Endocytic Pathway

The main difference between cytosolic pathway and endocytic pathway is that the cytosolic pathway possesses endogenous antigens, whereas the endocytic pathway possesses exogenous antigens.

Cytosolic and endocytic pathways are the two methods of antigen processing and presentation with MHC complex. Both pathways eliminate intracellular and extracellular antigens.

Key Areas Covered

1. What is Cytosolic Pathway
     – Definition, Features, Function
2. What is Endocytic Pathway
     – Definition, Features, Function
3. Similarities Between Cytosolic and Endocytic Pathway
     – Outline of Common Features
4. Difference Between Cytosolic and Endocytic Pathway
     – Comparison of Key Differences

Key Terms

Cytosolic Pathway, Endocytic Pathway

What is Cytosolic Pathway

The cytosolic pathway is the pathway by which the endogenous antigens undergo processing for the representation with MHC molecules. Generally, endogenous antigens undergo degradation to present with class I MHC molecules. This pathway is responsible for the normal turnover of intracellular proteins. In the cytosolic pathway, intracellular proteins undergo degradation into short peptides by the proteolytic systems in the cell. These proteolytic systems generally include proteins such as ubiquitin that form multifunctional protein complexes called proteosomes.

Figure 1: Cytosolic Pathway

Further, transporter proteins, including TAP, are responsible for the translocation of short peptides of the proteasome into the rough endoplasmic reticulum. This translocation requires ATP. The sizes of the short peptides translocated are 8-10 amino acids. Normally, it is the optimal peptide length for binding with class I MHC complex. Chaperone molecules aid the assembling of the peptides with class I MHC molecules.

What is Endocytic Pathway

The endocytic pathway is the pathway by which the exogenous antigens undergo processing and representation with class II MHC molecules. Generally, antigen-presenting cells internalize extracellular antigens by endocytosis or by phagocytosis. However, exogenous antigens undergo degradation into peptides within the compartments. Exogenous antigens undergoing the endocytic pathway usually take around 1-3 hours to appear on the cell membrane. Here, early endosomes contain high pH values, while late lysosomes have lowering pH. Low pH is important for the degradation reactions.

Figure 2: Endocytosis

Moreover, oligopeptides with 13-18 peptides bind to the class II MHC molecules. Meanwhile, endocytic compartments arrive at the cell periphery in order to fuse with the cell membrane. In general, the recycling of surface receptors occurs in this way.

Similarities Between Cytosolic and Endocytic Pathway

• Cytosolic and endocytic pathways are the two methods of antigen-presenting and processing with MHC molecules.
• Both pathways are responsible for eliminating intracellular and extracellular antigens.
• Both pathways form peptides and amino acids with exopeptidases.
• Moreover, T lymphocytes undergo both processes.

Difference Between Cytosolic and Endocytic Pathway

Definition

Cytosolic pathway refers to an immunological process that prepares antigens for presentation to special cells of the immune system called T lymphocytes, while the endocytic pathway refers to a pathway consisting of distinct membrane compartments, which internalize molecules from the plasma membrane and recycle them back to the surface or sort them to degradation.

Type of Antigens Processing

Cytosolic pathway processes endogenous antigens, while the endocytic pathway processes exogenous antigens.

Process

In the cytosolic pathway, endogenous antigens form cytoplasmic proteasome complexes to form peptides, while in the exocytic pathway, exogenous antigens undergo endocytosis to form endocytic compartments.

Endoplasmic Reticulum

Endogenous antigen complexes in the cytosolic pathway go into the endoplasmic reticulum, whereas exogenous antigen complexes bind to the cell membrane in the endocytic pathway.

Antigen Presenting

The cytosolic pathway presents antigens with class I MHC molecules, while the endocytic pathway presents antigens with class II MHC molecules.

Importance

Regular turnover of intracellular proteins occurs in the cytosolic pathway, while the endocytic pathway is responsible for the recycling of surface receptors.

Conclusion

In brief, cytosolic and endocytic pathways are the two methods of processing and presenting antigens with MHC molecules. They occur in T lymphocytes. Generally, the cytosolic pathway is an immunological process that possesses endogenous antigens with MHC class I molecules. These antigens form cytoplasmic proteasome complexes that go into the endoplasmic reticulum. Generally, the cytosolic pathway is responsible for the regular turnover of intracellular proteins. In comparison, the endocytic pathway possesses exogenous antigens with MHC class II molecules. However, these antigens undergo endocytosis and form peptides to represent antigens on the cytoplasm. However, the endocytic pathway is responsible for the recycling of surface proteins. Therefore, the main difference between the cytosolic and endocytic pathways is the type of antigens processed by each pathway.            

References:

1. Antigen processing and presentation with MHC – Davuniversity.org. (n.d.). Retrieved April 27, 2023

Image Courtesy:

1. “MHC Class I processing” By Scray – Own work (CC BY SA-3.0) via Commons Wikimedia
2. “Endocytosis types” By Mariana Ruiz Villarreal LadyofHats – Own Work (Public Domain) via Commons Wikimedia