Cloning of ubiquitin activating enzyme from wheat and expression of a functional protein in Escherichia coli

The initial step in the conjugation of ubiquitin to substrate proteins involves the activation of ubiquitin by ubiquitin activating enzyme, E1. Previously, we purified and characterized multiple species of E1 from wheat germ. We now describe the isolation and characterization of a cDNA clone encoding E1 from wheat. This clone (UBA1) was isolated from a cDNA expression library with anti-wheat E1 antibodies. It contained an open reading frame coding for 1051 amino acids and directed the synthesis of a protein that comigrated with a wheat germ E1 of 117 kDa. UBA1 was confirmed as encoding E1 by (i) comparison of the peptide map of the protein product of UBA1 synthesized in Escherichia coli with that of purified E1 from wheat, and (ii) amino acid sequence identity of peptides generated from purified E1 with regions of the derived amino acid sequence of UBA1. The isolation of two additional cDNAs closely related to UBA1 indicated that E1 was encoded by a small gene family in wheat. Nonetheless, a single poly(A+) mRNA size class of 4 kilobases hybridized with UBA1. When expressed in E. coli, the product of UBA1 catalyzed the formation of a thiol ester linkage between ubiquitin and an ubiquitin carrier protein. The ability of E. coli containing UBA1 to synthesize an active protein will allow us to identify domains important for E1 function using in vitro mutagenesis.