The Role of AHA Motifs in the Activator Function of Tomato Heat Stress Transcription Factors HsfA1 and HsfA2

Abstract

Using reporter assays in tobacco protoplasts and yeast, we investigated the function of the acidic C-terminal activation domains of tomato heat stress transcription factors HsfA1 and HsfA2. Both transcription factors contain short, essential peptide motifs with a characteristic pattern of aromatic and large hydrophobic amino acid residues embedded in an acidic context (AHA motifs). The prototype is the AHA1 motif of HsfA2, which has the sequence DDIWEELL. Our muta-tional analysis supports the important role of the aromatic and large hydrophobic amino acid residues in the core positions of the AHA motifs. The pattern suggests the formation of an amphipathic, negatively charged helix as the putative contact region with components of the basal transcription complex. In support of this concept, proline or positively charged residues in or adjacent to the AHA motifs markedly reduce or abolish their activity. Both AHA motifs of HsfA1 and HsfA2 contribute to activator potential, and they can substitute for each other; however, there is evidence for sequence and positional specificity.

Two different heat shock transcription factors regulate immediate early expression of stress genes in Arabidopsis

“…In contrast to class B and non-plant-HSFs, HSFs of classes A and C have an insertion of 21 and 7 amino acids, respectively, which separates the hydrophobic regions HR-A and B. A striking peculiarity of class B HSFs is the lack of AHA motifs in their CTAD, although these have been shown to be crucial for the activator function of class A HSFs (Do¨ring et al 2000). In contrast to class A HSFs, the transient overexpression of several HSFs belonging to class B was not sufficient for the activation of HS promoters in tobacco protoplasts (Czarnecka-Verner et al 1997, and class B HSFs fail to rescue the yeast hsf1 mutation (Boscheinen et al 1997).…”

The Heat Stress Transcription Factor HsfA2 Serves as a Regulatory Amplifier of a Subset of Genes in the Heat Stress Response in Arabidopsis

“…DNA fragments were purified by 'QIAquick gel extraction kit' (Qiagen). GUS reporter assays and analysis of protein expression in tobacco protoplasts were described previously (Scharf et al, 1998a;Do¨ring et al, 2000;Kotak et al, 2004). Protein extraction and analysis from Arabidopsis tissues were performed as described for tomato (Mishra et al, 2002).…”

Nuclear import and export of proteins in plants: a tool for the regulation of signalling

“…The two classes of plant HSF also contain dierent carboxy-terminal domains. Two or more acidic peptide motifs are found in class-A HSFs, which are essential for transcriptional activation (DoÈ ring et al 2000), whereas the carboxyterminal domains of class-B HSFs are neutral or basic (Czarnecka-Verner et al 2000).…”