Degradation of Aux/IAA proteins is essential for normal auxin signalling.
The growth substance auxin mediates many cellular processes, including division, elongation and differentiation. PSIAA6 is a member of the Aux/IAA family of short-lived putative transcriptional regulators that share four conserved domains and whose mRNAs are rapidly induced in the presence of auxin. Here PSIAA6 was shown to serve as a dominant transferable degradation signal when present as a translational fusion with firefly luciferase (LUC), with an in vivo half-life of 13.5 min in transgenic Arabidopsis seedlings. In a transient assay system in tobacco protoplasts using steady-state differences as an indirect measure of protein half-life, LUC fusions with full-length PSIAA6 and IAA1, an Aux/IAA protein from Arabidopsis, resulted in protein accumulations that were 3.5 and 1. 0%, respectively, of that with LUC alone. An N-terminal region spanning conserved domain II of PSIAA6 containing amino acids 18-73 was shown to contain the necessary cis-acting element to confer low protein accumulation onto LUC, while a fusion protein with PSIAA6 amino acids 71-179 had only a slight effect. Single amino acid substitutions of PSIAA6 in conserved domain II, equivalent to those found in two alleles of axr3, a gene that encodes Aux/IAA protein IAA17, resulted in a greater than 50-fold increase in protein accumulation. Thus, the same mutations resulting in an altered auxin response phenotype increase Aux/IAA protein accumulation, providing a direct link between these two processes. In support of this model, transgenic plants engineered to over-express IAA17 have an axr3-like phenotype. Together, these data suggest that rapid degradation of Aux/IAA proteins is necessary for a normal auxin response.
Worley
C K
CK
Section of Molecular and Cellular Biology, University of California at Davis, 1 Shields Avenue, Davis, CA 95616, USA.
Zenser
N
N
Ramos
J
J
Rouse
D
D
Leyser
O
O
Theologis
A
A
Callis
J
J
D017360:Arabidopsis; D029681:Arabidopsis Proteins; D004591:Electrophoresis, Polyacrylamide Gel; D007210:Indoleacetic Acids / Q000502:physiology*; D009687:Nuclear Proteins / Q000502:physiology*; D010940:Plant Proteins / Q000502:physiology*; D030821:Plants, Genetically Modified; D015398:Signal Transduction; D014157:Transcription Factors
C110728:AXR3 protein, Arabidopsis; D029681:Arabidopsis Proteins; D007210:Indoleacetic Acids; D009687:Nuclear Proteins; D010940:Plant Proteins; D014157:Transcription Factors