True regarding ubiquitin is: September 2009

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