Implicating a RING Finger E3 Ligase

Among other cellular responses, auxin signaling promotes lateral root growth in plants. Xie et al. identified a novel member of the C3HC4 RING finger E3 ligases of the SINA ( Drosophila ) and SIAH (human) family in Arabidopsis using a yeast two-hybrid screen for proteins that interacted with the transcriptional activator NAC1, a protein involved in transducing the auxin signal for lateral root growth. They called the protein SINAT5. In support of SINAT5 being a functional E3 ligase that could promote ubiqutination and degradation of substrates, SINAT5 interacted with the Arabidopsis member of the Ubc family, with ubiquitin, and with itself. Recombinant SINAT5 expressed in bacteria promoted the ubiquitination of itself and NAC1 in vitro. In plants, overexpression of SINAT5 led to reduced levels of NAC1 in roots and inhibited lateral root growth. Overexpression of a dominant negative mutant of SINAT5 led to increased levels of NAC1 and promoted lateral root growth. Another E3 ligase complex involved in auxin signaling is the SCF (Skp1/Cullin/F-box) complex. SCF mediates the ubiquitination of transcriptional repressors in response to auxin, promoting auxin signaling. Thus, ubiquitination appears to be involved in both promoting auxin signaling and terminating it. Q. Xie, H.-S. Guo, G. Dallman, S. Fang, A. M. Weissman, N.-H. Chua, SINAT5 promotes ubiquitin-realted degradation of NAC1 to attenuate auxin signals. Nature 419 , 167-170 (2002). [Online Journal]