Minisymposium 14: Protein Modification

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Abs # M1402: The RAD23 Family of Ubiquitin-Like Proteins Regulates Plant Development and Abscisic Acid Signaling in Arabidopsis thaliana

Presenter:	Farmer, Lisa M.       Contact Presenter
Authors	Farmer, Lisa M. (A)   Fu, Hong-Yong  (B)   Vierstra, Richard D. (A)
Affiliations:	(A): Department of Genetics, University Of Wisconsin-Madison, Madison, WI 53706 (B): Institute of Plant and Microbial Biology, Academia Sinica, Nankang, Taipei, Taiwan 11529

The Ubiquitin/26S Proteasome System (UPS) is the preferential mechanism by which transient regulatory or misfolded proteins are discarded. Ubiquitin (Ub) is covalently attached to target proteins by a series of E1-E2-E3 enzymatic activities, and conjugates are delivered to the 26S proteasome, where they are unfolded and degraded. Ub conjugation and proteolysis have been described, but less is known about the identities of specific targets and how they are transported to the proteasome. Evidence suggests that RAD23 proteins are responsible for delivering targets to the proteasome. RAD23 proteins have N-terminal Ub-like (UBL) and C-terminal Ub-associated (UBA) domains that facilitate their interactions with the proteasome and poly-Ub chains, respectively. We predict that these associations allow RAD23s to accept Ub conjugates and then dock with the proteasomal subunit RPN10 to relinquish their cargo. The Arabidopsis RAD23 family includes four highly conserved members. By analysis of genetic knockouts in the four AtRAD23 genes, we found that they are important for appropriate root, shoot and reproductive organ development in seedlings and mature plants, respectively. The AtRAD23s preferentially bind Lys48-linked poly-Ub chains, which are a signature for degradation. They also interact with RPN10, which helps regulate the levels of hormone signaling factors such as the ABA transcription factor ABI5. We have evidence that ABI5 levels are affected by multiple AtRAD23s. Further analysis should reveal other functions of the AtRAD23s, as well as the identities of important developmental regulators that are recycled by the UPS.