THE ARABIDOPSIS-THALIANA UBC7 13/14 GENES ENCODE A FAMILY OF MULTIUBIQUITIN CHAIN-FORMING E2 ENZYMES/

Covalent modification of proteins by attachment of multiubiquitin chai ns serves as an essential signal for selective protein degradation in eukaryotes. The specificity of ubiquitin-protein conjugation is contro lled in part by a diverse group of ubiquitin-conjugating enzymes (E2s or UBCs), We have previously reported that the product of the wheat Ta UBC7 gene recognizes ubiquitin as a substrate for ubiquitination in vi tro, catalyzing the condensation of free ubiquitin into multiubiquitin chains linked via lysine 48 (van Nocker, S., and Vierstra, R. D. (199 1) Proc. Natl. Acad. Sci. U. S. A. 88, 10297-10301), Based on this act ivity, this E2 may play a central role in the ubiquitin proteolytic pa thway by assembling chains in vivo. Here, we describe the cloning and characterization of a three-member gene family from Arabidopsis thalia na (designated AtUBC7/13/14) encoding structural homologs of TaUBC7. L ike TaUBC7, recombinant AtUBC7/13/14 proteins formed multiubiquitin ch ains in vitro. AtUBC7/13/14 mRNAs were found in all tissues examined, and unlike related UBCs from yeast, the levels of mRNA were not elevat ed by heat stress or cadmium exposure, Transgenic Arabidopsis were eng ineered to express increased levels of active AtUBC7, for the first ti me altering the level of an E2 in a higher eukaryote, Plants expressin g high levels of AtUBC7 exhibited no phenotypic abnormalities and were not noticeably enriched in multiubiquitinated conjugates. These findi ngs indicate that the in vivo synthesis of multiubiquitin chains is no t rate-limited by the abundance of AtUBC7 and/or involves other, yet u ndefined components.