In contrast to substrates of the SKP 1 Cullin 1 F box ( SCF ) complexes , in vitro ubiquitination of E2F1 by CUL 1 ROC1 ligase does not require E2F1 phosphorylation , is not stimulated by overexpression of F box protein SKP 2 , and is not affected by immunodepletion of SKP 1 or mutations in CUL 1 disrupting SKPI binding . ^^^