Which of the following statements about the ubiquitination of proteins are true?
1) ubiquitin is a relatively small protein that is widely expressed in eukaryotic cells.
2) it is covalently attached to target proteins in an atp-dependent reaction.
3) a carboxyl group of ubiquitin forms amide bonds with free amines found on the target protein.
4) target selection is determined by ubiquitin-protein ligases.

1) Ubiquitin is a relatively small protein that is widely expressed in eukaryotic cells.

2) It is covalently attached to target proteins in an ATP-dependent reaction.

4) Target selection is determined by ubiquitin-protein ligases.

Explanation:

Ubiquitin is a eukaryotic protein. It is very small in size and is highly conserved among the various species of eukaryotes. The function of ubiquitin is to mark an intracellular protein for degradation by proteasomes. Attachment of first ubiquitin to the target protein facilitates the joining of more ubiquitin molecules. The tagged protein enters proteolytic degradation. The ubiquitin molecules are covalently attached to a Lys residue in the target protein.

Ubiquitin specifically becomes covalently attached to the lysine residues of the target protein. This reaction is mediated by the ubiquitinating enzyme in an ATP dependent manner. Specific recognizing proteins recognize the target protein and facilitate the ligation of ubiquitin to target it for destruction by proteasomes.

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