IAA17/AXR3: Biochemical insight into an auxin mutant phenotype

The Aux/IAA genes are rapidly and specifically induced by the plant hormone auxin. The proteins encoded by this gene family are short-lived nuclear pr oteins that are capable of homodimerizing and heterodimerizing. Molecular, biochemical, and genetic data suggest that these proteins are involved in a uxin signaling. The pleiotropic morphological phenotype and altered auxin r esponses of the semidominant axr3-1 mutant of Arabidopsis result from a sin gle amino acid change in the conserved domain ii of the Aux/IAA protein IAA 17. Here, we show that the biochemical effect of this gain-of-function muta tion is to increase the half-life of the iaa17/axr3-1 protein by sevenfold. Intragenic mutations that suppress the iaa17/axr3-1 phenotype have been de scribed. The iaa17/axr3-1R3 revertant contains a second site mutation in do main I and the iaa17/axr3-1R2 revertant contains a second site mutation in domain III. Transient expression assays show that the mutant forms of IAA17 /AXR3 retain the ability to accumulate in the nucleus. Using the yeast two hybrid system, we show that the iaa17/axr3-1 mutation does not affect homod imerization. However, the iaa17/axr3-1 revertants counteract the increased levels of iaa17/axr3-1 protein by decreasing the capacity of the mutant pro tein to homodimerize. Interestingly, heterodimerization of the revertant fo rms of IAA17/AXR3 with IAA3/SHY2, another Aux/IAA protein, and ARF1 or ARF5 /MP proteins is affected only by changes in domain III. Collectively, the r esults provide biochemical evidence that the revertant mutations in the IAA 17/AXR3 gene affect the capacity of the encoded protein to dimerize with it self, other members of the Aux/IAA protein family, and members of the ARF p rotein family. By extension, these findings may provide insight into the ef fects of analogous mutations in other members of the Aux/IAA gene family.