Simplify the following text for a Grade 13 native English-speaking person.
Original: In the ATP-dependent ubiquitin\/proteasome pathway , particular proteins are targeted for destruction by the ligation of at least four copies of a small peptide called ubiquitin onto a substrate protein .
Simplified: In this system , particular proteins are targeted for destruction by the ligation of at least four copies of a small peptide called ubiquitin onto a substrate protein .