What is the Order of Protein Sorting by the Golgi Apparatus

What is the order of protein sorting by the Golgi apparatus? Proteins from the endoplasmic reticulum (ER) move into the ER-Golgi intermediate compartment, entering the Golgi apparatus from the cis Golgi network. Then, proteins progress to the medial and trans compartments in the Golgi apparatus. After that, proteins undergo sorting and move by direct molecular traffic into the lysosomes, plasma membrane, or cell exterior.

Protein sorting is a biological mechanism by which proteins transport to their appropriate destinations within or outside the cell. Therefore, many proteins destine into the ER, Golgi apparatus, lysosomes, and plasma membrane.  

Key Areas Covered

1. What is the Golgi apparatus
     – Definition, Features, Importance
2. What is Protein Sorting
     – Definition, Features, Importance
2. What is the Order of Protein Sorting by the Golgi Apparatus
     – Explanation of Steps

Key Terms

Golgi Apparatus, Protein Sorting

What is the Golgi Apparatus

The Golgi apparatus is an organelle occurring in the eukaryotic cells. It occurs inside the cytoplasm, a part of an endomembrane system. In addition, the main function of the Golgi apparatus is to package proteins into vesicles that are membrane-bound. These vesicles transport proteins to their different destinations in the cell. It receives proteins from the ER. Inside the Golgi apparatus, proteins undergo further processing and sorting. Moreover, the final destinations of proteins can be either lysosomes, plasma membranes, or secretion. Glycolipids and sphingomyelin synthesis occur inside the Golgi apparatus. In addition, complex polysaccharides of the cell wall are synthesized inside the Golgi apparatus in plant cells. Therefore, the Golgi apparatus processes a broad range of cellular constituents.

Figure 1: The Golgi Apparatus

Morphologically, the Golgi apparatus contains flattened sacs or cisternae that are enclosed by the plasma membrane. They serve as multiple discrete compartments. It also contains associated vesicles. Apart from that, the main feature of the Golgi apparatus is the presence of polarity, both structural and function-wise. Therefore, the cis face is the entry face for proteins, lipids, and carbohydrates. The cis face is convex while orienting towards the nucleus. Then, these cellular constituents transport through the medial Golgi apparatus, exiting from the exit face that is the trans face. Here, the trans face of the Golgi apparatus is concave in shape.    

What is Protein Sorting

Protein sorting or protein targeting is a distinct event occurring inside the Golgi apparatus. The function of the Golgi apparatus is to transport proteins to their destinations either inside the cell or outside the cell. Signal peptides serve as the targeting signals for the cellular transport machinery important to direct proteins to their specific destinations within or outside the cell. Signal peptides do not contain consensus sequences. Meanwhile, the three main structural features of a signal peptide include a positively charged, hydrophilic region near the N-terminus, spanning 10 to 15 hydrophobic amino acids at the middle, and a slightly polar region near the C-terminus. However, signal peptidase is the cleaving enzyme of the signal peptide occurring when the protein arrives at its destination.

Figure 2: Protein Translocation

Furthermore, the translocation of proteins occurs in two methods: co-translational translocation and post-translational translocation. Co-translational translocation is the type of translocation that occurs during translation. Most secretory and membrane-bound proteins undergo this type of translocation. Proteins that reside Golgi apparatus, ER, and endosomes use this pathway. Here, the process of translocation begins while the protein is under translation with the identification of the signal peptide at the N-terminus of the nascent protein. Therefore, the ribosome-protein complex transports into the ER in co-translational translocation. In comparison, secretory proteins undergo post-translational translocation. There, some proteins undergo translation inside the cytosol and, later, transport into the ER. Moreover, proteins translocate to chloroplasts, mitochondria, and peroxisomes undergo post-translational translocation.

What is the Order of Protein Sorting by the Golgi Apparatus

After the translation by ribosomes, proteins undergo translocation, transporting to their destinations in order to undergo specific functions of them. Therefore, co-translationally or post-translationally, proteins transport into the ER. After that, these proteins transport to the intermediate compartment of the ER-Golgi apparatus. Next, they enter the cis face of the Golgi apparatus. Here, the Golgi apparatus is the compartment in which proteins undergo distinct processing and sorting events. The Golgi apparatus contains multiple discrete compartments for its function. Later, proteins progress into the medial compartments of the Golgi apparatus. Finally, proteins move to the trans face of the Golgi apparatus. Importantly, it serves as the sorting and distribution center of proteins. In addition, the trans face of the Golgi apparatus directs the proteins to the lysosomes, plasma membrane, or for secretion.

Conclusion

In brief, the Golgi apparatus is the organelle that occurs in the eukaryotic cells. Its function is to modify and distribute molecules such as proteins, carbohydrates, and lipids. In addition, it contains multiple discrete compartments. Proteins enter into the cis face of the Golgi apparatus from the ER-Golgi intermediate compartment. Then, proteins move to the medial and trans compartments. Meanwhile, the trans face of the Golgi apparatus serves as the sorting and distribution center of proteins. Therefore, proteins are directed to either lysosomes, plasma membrane, or for secretion by the trans face of the Golgi apparatus.

References:   

1. Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. The Golgi Apparatus.
2. Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Chapter 9, Protein Sorting and Transport – The Endoplasmic Reticulum, Golgi Apparatus, and Lysosomes.

Image Courtesy:

1. “Golgi apparatus (borderless version)-en” By Kelvinsong – Derivative work (CC-BY 3.0) via Commons Wikimedia
2. “General Overview of Protein Targeting” By Neeko Sneako2 – Own Work (CC-BY-SA 4.0) via Commons Wikimedia