One-Two Punch to Ethylene Production

Abstract

Ethylene gas functions as a hormone in plants and is produced in response to stress or the growth regulator cytokinin. The rate-limiting step of ethylene biosynthesis is catalyzed by the enzyme 1-aminocyclopropane-1-carboxylic acid synthase (ACS), and in some instances, ethylene production is regulated through control of ACS expression. However, posttranscriptional regulation also appears to be important, and Wang et al. have clarified a new mechanism by which such regulation may occur. They characterized eto1 , a gene that decreases ACS activity and production of ethylene. The association of ETO1 protein with ACS was confirmed by yeast two-hybrid assays, in vitro experiments, and immunoprecipitation from plant cells. In vitro, ETO1 directly inhibited ACS activity. But ETO also contains a BTB (Broad-complex, Tramtrack, Bic-a-brac) domain that is associated with adaptor proteins that link substrates to ubiquitin ligase complexes that mark proteins for proteasomal degradation. ETO1 interacted with the CUL3A protein, which functions as a scaffold in ubiquitin E3 ligase complexes. Overexpression of ETO1 in cultured cells caused increased degradation of ACS, an effect that was blocked by a proteasome inhibitor, MG132. Thus, the authors propose that ETO1 has a two-pronged inhibitory effect on ACS, directly inhibiting the enzyme's activity and also targeting ACS for degradation. K. L.-C. Wang, H. Yoshida, C. Lurin, J. R. Ecker, Regulation of ethylene gas biosynthesis by the Arabidopsis ETO1 protein. Nature 428 , 945-950 (2004). [Online Journal]