SCF ( beta TRCP ) and phosphorylation dependent ubiquitinationof 1 kappa B alpha catalyzed by Ubc 3 and Ubc 4 . ^^^ We demonstrate specific ubiquitination of 1 kappa B alpha by Ubc 3 and Ubc 4 in a phosphorylation and SCF beta TRCP dependent manner and that both are capable of associating with the SCF beta TRCP complex isolated from human cells . ^^^