Ubiquitin

There are two major pathways of protein degradation in eukaryotes.

One involves lysosomal proteases and does not require ATP. The other way involves ubiquitin and is ATP-dependent. Ubiquitin is a small, highly-conserved regulatory protein that is ubiquitously expressed in eukaryotes.

Ubiquitination (or ubiquitylation) refers to the post-translational modification of a protein by the covalent attachment (via an isopeptide bond) of one or more ubiquitin monomers. The most prominent function of ubiquitin is labeling proteins for proteasomal degradation.

Ans. D: All of the above

Ubiquitin (Ub) is a small protein that is composed of 76 amino acids.

This protein is found only in eukaryotic organisms and is not found in either eubacteria or archaebacteria. Ub is a heat-stable protein that folds up into a compact globular structure.

It is found throughout the cell (thus, giving rise to its name) and can exist either in free form or as part of a complex with other proteins.

Single Ub molecules can be conjugated to the lysine of these proteins, or more commonly, Ub-chains can be attached. Ub is involved in many cell processes.

Ub is conjugated to the protein cyclin during the G1 phase of mitosis and thus plays an important role in regulating the cell cycle.

Ub conjugation is also involved in DNA repair, embryogenesis, the regulation of transcription, and apoptosis (programmed cell death).