In the nucleus (Grenfell et al. 1994), the UBA1-ubiquitin thiol ester conjugate transfers ubiquitin from UBA1 to an internal cysteine residue of the E2 enzyme, forming a thiol ester conjugate between ubiquitin and the cysteine residue of E2 (Jin et al. 2007, inferred from the rabbit homologue in Hershko et al. 1983, reviewed in Groettrup et al. 2008). The E2 then disengages from UBA1 (Eletr et al. 2005).