True regarding ubiquitin is:

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 ( an isopeptide bond) of one or more ubiquitin monomers. The most prominent function of ubiquitin is labeling proteins for proteasomal degradation.