This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Proteins targeted for destruction in eukaryotes are covalently linked to : 
Select and explain. 
A. Clathrin, B. Pepsin, C. Laminin, D. Ubiquitin
D: Ubiquitin
Answer: D: Ubiquitin 
Explanation: D i.e. UbiquitinUbiquitine is a small, highly conserved protein (76 amino acid polypeptide), that plays a key role in marking (targeting) & escoing many intracellular protein mainly abnormal newly produced protein and aged normal protein for degradationQ (ticket of degradation) in multisubunit proteolytic paicles the 26 S proteasome. It is also involved in activation of NFkB, cell cycle regulation (degradation of cycles), DNA repair, viral infections etc.Clathrin is a peripheral membrane protein, that helps in endocytosis. Pepsin is proteolytic enzyme secreted by chief cells of stomach and is used for digestion. Laminin is found in extracellular matrix a most abundant glycoprotein present in basement membrane