Answer:
a) Ubiquitin is a relatively small protein that is widely expressed in eukaryotic cells.
b) It is covalently attached to target proteins in an ATP-dependent reaction. d) Target selection is determined by ubiquitin-protein ligases.
Explanation:
Ubiquitin is a small and highly conserved eukaryotic protein. It is involved in targeting an intracellular protein for degradation by proteasomes. For the purpose, ubiquitin is ligated to the target protein that is to enter proteolytic degradation. The ubiquitin molecules are covalently attached to a Lys residue in the target protein.
Ligation of ubiquitin to the lysine residue of the target protein is mediated by a ubiquitinating enzyme which in turn uses ATP as a source of energy. For example, inactivation of the active cyclin-CDK complex is carried out by ligation of several molecules of ubiquitin. The ligation is triggered by specific recognizing proteins that recruit the ubiquitin ligase to add ubiquitin to the cyclin-CDK complex to target it for destruction by proteasomes.
