Well, because we need to fit the protein into the 20s core. And so it's the 19s regulatory subunit that binds to the polyubiquitinated protein. It begins using ATP molecules to basically unfold that protein and insert that protein into the 20s core. And in the 20s core is where we begin to break down the protein into smaller peptides that consist of about seven to nine amino acids in length. And also what happens inside a 20s protein is we have an enzyme known as Isappeptidase that begins to remove the ubiquitin molecules. Because the ubiquitant molecules are not broken down our cells reuse the ubiquitan molecules over and over and over.