A ubiquitin ligase is a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that in combination with an E2 ubiquitin-conjugating enzyme

Ubiquitin-conjugating enzyme

Ubiquitin-conjugating enzymes, also known as E2 enzymes and more rarely as ubiquitin-carrier enzymes, perform the second step in the ubiquitination reaction that targets a protein for degradation via the proteasome.The ubiquitination process covalently attaches ubiquitin, a short protein of 76...

 causes the attachment of ubiquitin

Ubiquitin

Ubiquitin is a small regulatory protein that has been found in almost all tissues of eukaryotic organisms. Among other functions, it directs protein recycling.Ubiquitin can be attached to proteins and label them for destruction...

 to a lysine

Lysine

Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....

 on a target protein via an isopeptide bond

Isopeptide bond

An isopeptide bond is an amide bond that is not present on the main chain of a protein. The bond forms between the carboxyl terminus of one protein and the amino group of a lysine residue on another protein....

; the E3 ubiquitin ligase targets specific protein substrates for degradation by the proteasome

Proteasome

Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria.  In eukaryotes, they are located in the nucleus and the cytoplasm.  The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...

. In general, the ubiquitin ligase is involved in polyubiquitination: A second ubiquitin is attached to the first, a third is attached to the second, and so forth. Polyubiquitination marks proteins for degradation by the proteasome.

However, there are some ubiquitination events that are limited to mono-ubiquitination, in which only a single ubiquitin is added by the ubiquitin ligase to a substrate molecule.