TheCell7e Ch11 Lecture

Overview of Protein Sorting and Transport

Eukaryotic cells are characterized by their complexity, featuring membrane-enclosed organelles that are particularly evident in the cytoplasm. This structural subdivision allows for efficient cellular function, compensating for their significantly larger volumes in comparison to prokaryotic cells. Key organelles involved in protein sorting and transport include the Endoplasmic Reticulum (ER), Golgi Apparatus, lysosomes, and various vesicular transport mechanisms. These structures facilitate the sorting and targeting of proteins, ensuring that they are delivered to their appropriate destinations.

The Endoplasmic Reticulum (ER)

The Endoplasmic Reticulum is an extensive network of membrane-enclosed tubules and sacs, known as cisternae, that proliferates throughout the cytoplasm. It is the largest organelle in eukaryotic cells and is integral to protein processing and sorting.

Structure and Domains

The ER exhibits two main domains:

• Rough ER: Studded with ribosomes on its outer membrane, playing a pivotal role in protein synthesis and processing.

• Smooth ER: Lacking ribosomes and mainly involved in lipid metabolism.

Functionality:

The importance of the ER in protein sorting was established through pioneering experiments by George Palade and his team in the 1960s, utilizing pancreatic acinar cells. These studies highlighted the sequence of protein synthesis, labeling, and tracking through the ER, Golgi, and secretion pathways. The defined secretory pathway can be summarized as:

• Rough ER → Golgi Apparatus → Secretory Vesicles → Cell Exterior

Protein Synthesis and Sorting

The synthesis of proteins on free ribosomes leads to their retention in the cytosol or transport to nucleus and organelles. In contrast, proteins synthesized on ribosomes attached to the membrane are directly translocated into the ER. Signal sequences at the amino terminus facilitate the targeting of ribosomes to the ER. Once the polypeptide chain enters the ER, the signal is cleaved.

Protein Translocation Mechanisms

Proteins can be translocated into the ER through either co-translational targeting, where the synthesis and entry into the ER occur simultaneously, or post-translational targeting, which is often the case in yeast cells. In both cases, chaperone proteins, such as BiP, assist in the proper folding during the translocation process.

The Golgi Apparatus

The Golgi apparatus functions as a central hub for protein processing, sorting, and distribution. Proteins transported from the ER are modified, packaged, and dispatched to various destinations, including lysosomes, plasma membranes, or for secretion.

Structure of Golgi Apparatus

The Golgi is organized into four compartments:

• Cis Compartment: Receives proteins from the ERGIC.

• Medial and Trans Compartments: Major sites for protein modifications.

• Trans-Golgi Network: Functions as a sorting site.

Protein Movement Models

There exist two prominent models explaining the mechanisms of protein movement through the Golgi:

• Stable Cisternae Model: Suggests that proteins are transported between fixed cisternae via vesicles.

• Cisternal Maturation Model: Proposes that cisternae themselves progressively mature to transport proteins in a cis-to-trans direction.

Glycosylation Processes

In the Golgi, glycoproteins undergo further modifications of their carbohydrate portions, leading to specific alterations such as N-linked glycosylation. Lysosomal proteins receive additional mannose-6-phosphate groups that facilitate their sorting and delivery to lysosomes.

Lysosomes

Lysosomes serve as the cell's digestive system, containing numerous enzymes to digest biological polymers. Sufficiently maintained acidic conditions (pH 5) ensure that these enzymes remain inactive in the cytosol, preventing unwanted degradation of cellular components.

Formation and Function of Lysosomes

Lysosomes form when transport vesicles originating from the Golgi apparatus fuse with late endosomes. They digest material absorbed via endocytosis and are responsible for the degradation of large particles, including pathogens by phagocytes through phagocytosis.

Autophagy

This process involves the degradation of internal cellular components, effectively recycling nutrients during stress or starvation. During apoptosis, autophagy assists in cellular turnover and can be regulated depending on cellular requirements.

The Vesicular Transport Mechanism

Vesicular transport is critical for maintaining the functional organization within cells. Vesicles selectively recognize and fuse with proper target membranes facilitated by multiple protein interactions. Key components include:

• Coat Proteins: Such as COPII (ERGIC to Golgi) and Clathrin (Golgi to endosomes or plasma membrane).

• SNAREs: Mediate the docking and fusion process by engaging with specific target membranes, ensuring the correct delivery of vesicular contents.

Conclusion

Understanding the complex processes involved in protein sorting and transport is key to grasping cellular functionality within eukaryotic cells. These mechanisms ensure that proteins and lipids are not only synthesized but also properly folded, post-translationally modified, and directed to their ultimate destinations, thus supporting cellular homeostasis and function.