Question

1) : Briefly describe two entirely distinct processes involving protein degradation that are of central importance for the functioning of eukaryotic cells that form the body of animals. In your answer include a brief description of the key components involved in these processes. Also include a brief description of how these processes aid in the overall function of the organism.

(2) : In the process of intracellular membrane vesicle transport in Eukaryotes neither the formation of such transport vesicles nor their fusion with a target membrane occur spontaneously in the cell. Briefly describe the machinery that cells use to achieve these two processes.

(3): Describe the design of the DNA sequence of an artificial chromosome in a typical eukaryote for expressing an engineered transmembrane protein in the plasma membrane. In your answer list the 9 sequence features this DNA fragment requires to work, briefly state the function of each, and briefly indicate what would go wrong if that sequence feature was missing.

Answer 1

1)The Ubiquitin-Proteasome Pathway

The main pathway of selective protein degradation in eukaryotic cells involves ubiquitin as a marker that targets cytosolic and nuclear proteins for rapid proteolysis. Ubiquitin is a 76 amino acid polypeptide that is highly conserved in all eukaryotes. Proteins are marked for degradation by the attachment of ubiquitin to the amino group of the side chain of a lysine residue. Additional ubiquitins are then added to make a multiubiquitin chain. Such polyubiquitinated proteins are recognized and degraded by a multisubunit protease complex called the proteasome. Ubiquitin is released in the process, so it can be reused in another cycle. As the attachment of ubiquitin marks proteins for rapid degradation, the stability of many proteins is determined by whether they become ubiquitination. Thus this pathway is important for the regulation of major processes within the body.

Lysosomal Proteolysis

Another major pathway of protein degradation involves the uptake of proteins by lysosomes. Lysosomes contain digestive enzymes, including several proteases. They have several roles in cell metabolism, which includes digestion of extracellular proteins taken up by endocytosis as well as the gradual turnover of cytoplasmic organelles and cytosolic proteins. The array of proteases and other digestive enzymes in lysosomes prevents uncontrolled degradation of the contents of the cell. Therefore, in order to be get degraded by lysosomal proteolysis, cellular proteins must first be taken up by lysosomes. One pathway for this uptake of cellular proteins called autophagy, involves the formation of vesicles in which small area of cytoplasmic organelles are enclosed in membranes derived from the endoplasmic reticulum. These vesicles get fused with lysosomes, and then degradative lysosomal enzymes digest them. The uptake of proteins into autophagosomes appears to be nonselective, so it results in the eventual slow degradation of long-lived cytoplasmic proteins. Not all protein uptake by lysosomes is nonselective. For example, lysosomes are able to take up and degrade certain cytosolic proteins in a selective manner as a response to cellular starvation.