The secretory & endocytic pathways 2023 draft

Overview of Secretory and Endocytic Pathways

This lecture addresses the mechanisms of protein trafficking within secretory and endocytic pathways, elucidating how proteins exit the endoplasmic reticulum (ER), navigate to their final destinations, and how mislocated ER proteins can be retrieved. It will also cover the process of endocytosis for plasma membrane proteins and extracellular materials, as well as examine the roles of these pathways in human diseases.

General Principles of Trafficking

Outline of Pathways

The secretory and endocytic pathways involve a series of cellular compartments:

• Endoplasmic Reticulum (ER)

• Golgi Apparatus

  • Cis-Golgi

  • Medial-Golgi

  • Trans-Golgi Network (TGN)

• Plasma Membrane

• Lysosomes

• Endosomes (both early and late)

Transport Mechanisms

Transport through these pathways typically relies on the formation of vesicles. Key components involved in vesicle formation and cargo recruitment include:

• Protein Coats: Clathrin, COPI, and COPII.

• Rab Proteins: Assist in membrane fusion by binding tethering proteins.

• SNARE Proteins: Facilitate the fusion of vesicles with target membranes.

ER to Golgi Transport

Overview

Proteins synthesized in the ER are packaged into COPII vesicles for export. Once vesicles form, they undergo uncoating and fuse with each other to create vesicular tubular clusters, which then deliver proteins to the Golgi apparatus.

COPII Vesicle Formation

• Sec12 catalyzes GDP-GTP exchange on Sar1, prompting it to associate with the ER membrane, which in turn recruits Sec23-Sec24.

• Cargo proteins are incorporated into vesicles via adaptor proteins such as Sec24. Once fully assembled, the vesicle buds off from the ER.

Cargo Recruitment

Efficient export from the ER requires specific export signals recognized by Sec24. Notably, the glycoprotein ERGIC-53 serves as a crucial receptor aiding in the retrieval of soluble proteins lacking intrinsic export signals.

Human Disease and ER Transport Defects

Defects in ER export are implicated in several disorders, one example being Cranio-lenticulo-sutural dysplasia (CLSD). This autosomal recessive syndrome arises from mutations in Sec23a, causing:

• Dilation of the ER

• Reduced capability to generate cargo-containing vesicles

• Decreased export of collagen from the ER

Retrieval of ER Proteins

From the Golgi

ER proteins may be non-selectively packaged in COPII vesicles but often contain specific ER retrieval motifs to facilitate their return from the Golgi. For instance, ERGIC-53 is retrieved via COPI vesicles.

Membrane Proteins

• Many ER resident membrane proteins possess a dilysine (KKXX) ER retrieval motif. The C-terminus of ERGIC-53 includes a related retrieval motif (KKFF).

• Proteins carrying the KKXX motif are retrieved through interaction with the COPI coat complex.

Luminal Proteins

Many soluble proteins have a KDEL retrieval motif (e.g., BiP) recognized by a KDEL receptor, which is crucial for their return from the Golgi to the ER.

Case Study: Adenovirus and MHC Class I

Adenovirus exploits the ER retrieval pathway by utilizing the integral membrane protein E3/19K, which binds MHC class I. The E3/19K protein contains an ER retrieval motif (KKXX), allowing it to be sequestered away and preventing MHC class I from reaching the cell surface, thereby evading immune detection.

Golgi Apparatus Functionality

Structure

The Golgi apparatus, noted for its stacked membrane organization (cisternae), is pivotal for further protein processing and sorting after ER export.

Protein Trafficking

Proteins transition through the Golgi from the cis to the trans side. There is ongoing debate over whether this occurs via stack maturation or vesicle-mediated transport.

Modification Processes

Within the Golgi, glycosylation modifies N-linked oligosaccharides to enhance protein solubility and stability, while also facilitating molecular recognition through further structural changes.

Trans-Golgi Network Sorting

The TGN acts as a sorting hub for proteins based on whether they require specific signals to adjust their destinations:

• Plasma membrane proteins typically do not require specific signals for secretion.

• Clathrin-coated vesicles form for proteins destined for lysosomes or other organelles.

Mannose-6-Phosphate as a Sorting Signal

Mannose-6-phosphate (M6P) acts as a key signal for the sorting of lysosomal enzymes. Proteins bearing M6P are recognized by M6P receptors, allowing their transit to early endosomes. Disruption of this process can lead to diseases such as I-cell disease, indicative of lysosomal storage disorders.

Endocytic Pathway Insights

Overview

Endocytosis involves the uptake of extracellular materials through vesicles formed from the plasma membrane, delivering contents to early endosomes for sorting.

Receptor Dynamics

• LDL receptors take up lipoproteins, recycling back to the surface after releasing their ligands.

• EGF receptors illustrate signaling regulation by being internalized and degraded to terminate signaling over time.

Clathrin-Mediated Endocytosis

Clathrin plays a crucial role in receptor-mediated endocytosis, recruiting various receptors and adapting proteins to form clathrin-coated vesicles for cellular uptake.

Sorting Mechanisms in Endosomes

Recycling vs. Degradation

Recycling endosomes return receptors to the plasma membrane, while those designated for degradation navigate to late endosomes and lysosomes through multivesicular bodies (MVBs).

Viruses and Endocytosis

Many viruses, including Hepatitis C, utilize clathrin-mediated endocytosis to enter cells by engaging specific receptors for their entry and subsequent release of their genetic material into the cytoplasm.

Conclusion

The secretory and endocytic pathways are vital for cellular function and integrity, governed by precise sorting mechanisms at various stages. Genetic mutations or pathogen exploitation of these pathways can lead to significant human diseases and highlight the importance of further research in therapeutic interventions.

References

• Borgese, N. (2016). Getting membrane proteins on and off the shuttle bus between the endoplasmic reticulum and the Golgi complex. J Cell Sci, 129: 1537-1545.

• Guo et al. (2014). Protein sorting at the trans-Golgi Network. Annual Review of Cell and Developmental Biology, 30: 169-206.

• Kaksonen, M. & Roux, A. (2018). Mechanisms of clathrin-mediated endocytosis. Nature Reviews Molecular Cell Biology, 19: 313-326.

• Elkin, S.R., Lakoduk, A.M., & Schmid, S.L. (2016). Endocytic pathways and endosomal trafficking: a primer. Wiener Medizinische Wochenschrift, 166: 196–204.

• Zeisel et al. (2011). Molecular mechanisms and targets for antiviral therapies. Journal of Hepatology, 54: 566-576.