Question

Compare the processing of cellular proteins with that of proteins secreted by a cell.

Short answer

In summary, both cellular and secreted proteins are synthesized on ribosomes, but secreted proteins are specifically synthesized on ribosomes associated with the RER. Processing, such as folding and post-translational modifications, occurs for both types of proteins, with secretory proteins undergoing more extensive processing in the ER and Golgi apparatus. Cellular proteins are transported to various cellular compartments, while secretory proteins follow a specific pathway involving the Golgi apparatus and the trans-Golgi network, ultimately leading to exocytosis for secreted proteins.

Step-by-step solution

Synthesis of proteins

Both cellular proteins and secreted proteins are synthesized in the ribosomes. However, the synthesis of secretory proteins occurs at the ribosomes associated with the rough endoplasmic reticulum (RER) while the synthesis of cellular proteins may occur at free ribosomes or ribosomes associated with RER.

Processing of proteins

Both types of proteins undergo processing, such as folding and post-translational modifications (e.g., glycosylation, phosphorylation, or disulfide bond formation). But the degree and type of processing vary between cellular proteins and secreted proteins. Secretory proteins generally undergo more extensive processing and modifications in the endoplasmic reticulum (ER) and the Golgi apparatus. They are packaged into vesicles and sorted for secretion.

Transport of proteins within the cell

For cellular proteins, after processing, they are transported to their respective cellular compartments or locations, such as the nucleus, mitochondria or cytosol. The transport proteins in the cytoplasm and the nucleus play a crucial role in this process. Secretory proteins undergo a specific pathway after processing, where they are transported to the Golgi apparatus for further modifications and sorting. From there, they move to the trans-Golgi network, where they are packaged into secretory vesicles.

Secretion of proteins out of the cell

For cellular proteins, they remain within the cell and carry out their designated functions. In contrast, secretory proteins are exocytosed or released outside the cell when the secretory vesicles fuse with the plasma membrane. The proteins are then either secreted into the extracellular matrix or become a part of the cell membrane as receptor proteins or channels. In conclusion, both cellular and secreted proteins undergo synthesis, processing, and transportation within the cell. However, the processing and transportation pathways differ significantly, with secretory proteins following a more defined route that ultimately leads to their release outside the cell or insertion into the cell membrane.

Key concepts

Ribosome Function

Ribosomes are essential cellular machines that play a key role in the process of protein synthesis. These tiny yet complex structures are often referred to as the 'workbenches' where proteins are assembled.

Functionally, ribosomes can be found either floating freely within the cytoplasm or bound to the rough endoplasmic reticulum (RER). The location of a ribosome is indicative of the protein's destination: free ribosomes primarily synthesize proteins that will function within the cytosol, whereas RER-bound ribosomes create proteins that are destined for secretion or for use within the cell's membrane systems.

During protein synthesis, ribosomes translate the genetic instructions encoded by messenger RNA (mRNA) into polypeptide chains. These chains undergo subsequent folding and modifications to become functional proteins. Ribosomes themselves are composed of ribosomal RNA and proteins, and consist of two subunits that must come together to begin the process of translation. Understanding ribosome function is pivotal in appreciating how proteins are created and how they contribute to the vast array of cellular functions.

Post-translational Modifications

Once a protein is synthesized, it often goes through a series of modifications known as post-translational modifications (PTMs). These changes are vital for the protein to acquire its fully functional three-dimensional structure and to perform its specific role within the cell.

PTMs can include the addition of functional groups, such as phosphate (phosphorylation) or sugar (glycosylation), the alteration of amino acid side chains, and the formation of disulfide bonds that stabilize protein structure. These modifications impact various protein characteristics including stability, activity, localization, and interaction with other molecules.

Glycosylation and Its Importance

Glycosylation, the attachment of sugar moieties to proteins, is particularly significant for secretory proteins. This process can be crucial for protein stability and for recognition by other cells or signaling molecules. Misfolded or improperly modified proteins can lead to cellular dysfunction or diseases, which underlines the significance of precise PTMs.

Protein Secretion Pathway

The protein secretion pathway is a sophisticated process that transports proteins from their site of synthesis to the outside of the cell, or into cell membranes. This pathway begins with the synthesis of precursor proteins on the RER-bound ribosomes. The proteins are then inserted into the RER where initial modifications, such as folding and glycosylation, occur.

Subsequently, the proteins are transported to the Golgi apparatus in vesicles. Within the Golgi, they undergo further modifications including precise cutting, folding, and addition of sugars. The mature proteins are then sorted and packed into secretory vesicles. The final step in the pathway is exocytosis, where these vesicles fuse with the plasma membrane to release the protein into the extracellular space or to integrate it into the cell membrane.

The protein secretion pathway is particularly crucial for the export of hormones, enzymes, and antibodies, all of which are pivotal for the proper functioning of an organism. This pathway exemplifies the careful orchestration within a cell to maintain balance and respond to the cell’s environment.