Ub-Cys632-UBA1 adenylates ubiquitin in the cytosol

Activation of ubiquitin by UBA1 proceeds through 3 steps: adenylation of ubiquitin, conjugation of ubiquitin from adenyl-ubiquitin to an internal cysteine residue of UBA1, and adenylation of a second molecule of ubiquitin. In the third step, the UBA1-ubiquitin conjugate adenylates the C-terminal glycine of a second ubiquitin molecule. This results in UBA1 loaded with 2 ubiquitin molecules, one of which is conjugated to an internal cysteine residue of UBA1 and one of which is adenylated and non-covalently bound to UBA1 (Jin et al. 2007, inferred from the rabbit homologue in Haas et al. 1982, Hershko et al. 1983, reviewed in Groettrup et al. 2008).

泛素化过程的激活由 UBA1 通过三个步骤完成：首先，泛素发生腺苷化；其次，泛素从腺苷-泛素转移到 UBA1 的内部半胱氨酸残基进行共价连接；最后，第二个泛素分子发生腺苷化。在第三步中，UBA1-泛素共价复合物将第二个泛素分子的羧基端甘氨酸腺苷化。这一过程使得 UBA1 结合了两个泛素分子，其中一个与 UBA1 的内部半胱氨酸残基共价连接，另一个则被腺苷化并以非共价形式与 UBA1 结合（Jin 等人，2007 年，根据 Haas 等人（1982 年）、Hershko 等人（1983 年）的研究推断，Groettrup 等人，2008 年综述）。