Antibodies for ubiquitinated proteins

A method of detecting a site of a diglycine (Gly-Gly) epitope linked to the epsilon amino group of a lysine in a protein present in a sample, wherein the method comprises

• (i) cleaving protein(s) in the sample with a protease which cleaves at the C-terminus of Arg to form a mixture of cleavage peptides, wherein at least one cleavage peptide in the mixture comprises a lysine which is linked through its epsilon amino group to the C-terminus of a Gly-Gly dipeptide;
• (ii) contacting the mixture of cleavage peptides with an antibody, wherein said antibody specifically binds to a Gly-Gly dipeptide linked to the epsilon amino group of a lysine, and said contacting is performed under conditions that allow said antibody to specifically bind to said at least one cleavage peptide;
• and
• (iii) isolating said at least one cleavage peptide bound to said antibody from the mixture of cleavage peptides, thereby detecting a site of a diglycine (Gly-Gly) epitope linked to an epsilon amino group of a lysine in a protein present in said sample.