tokens: In this study , we demonstrate reconstitution of a cell-free processing system and demonstrate directly that : ( a ) the ubiquitin -proteasome system is involved in processing of the intact p105 precursor , ( b ) conjugation of ubiquitin to the precursor is an essential intermediate step in the processing , ( c ) the recently discovered novel species of the ubiquitin-carrier protein , E2-F1 , that is involved in the conjugation and degradation of p53 , is also required for the limited processing of the p105 precursor , and ( d ) a novel , approximately 320-kDa species of ubiquitin-protein ligase , is involved in the process .