Role of Hsp17.4-CII as Coregulator and Cytoplasmic Retention Factor of Tomato Heat Stress Transcription Factor HsfA2

Abstract

HsfA2 is a heat stress (hs)-induced Hsf in peruvian tomato (Lycopersicon peruvianum) and the cultivated form Lycopersicon esculentum. Due to the high activator potential and the continued accumulation during repeated cycles of heat stress and recovery, HsfA2 becomes a dominant Hsf in thermotolerant cells. The formation of heterooligomeric complexes with HsfA1 leads to nuclear retention and enhanced transcriptional activity of HsfA2. This effect seems to represent one part of potential molecular mechanisms involved in its activity control. As shown in this paper, the activity of HsfA2 is also controlled by a network of nucleocytoplasmic small Hsps influencing its solubility, intracellular localization and activator function. By yeast two-hybrid interaction and transient coexpression studies in tobacco (Nicotiana plumbaginifolia) mesophyll protoplasts, we found that tomato (Lycopersicon esculentum) Hsp17.4-CII acts as corepressor of HsfA2. Given appropriate conditions, both proteins together formed large cytosolic aggregates which could be solubilized in presence of class CI sHsps. However, independent of the formation of aggregates or of the nucleocytoplasmic distribution of HsfA2, its transcriptional activity was specifically repressed by interaction of Hsp17.4-CII with the C-terminal activator domain. Although not identical in all aspects, the situation with the highly expressed, heat stress-inducible Arabidopsis HsfA2 was found to be principally similar. In corresponding reporter assays its activity was repressed in presence of AtHsp17.7-CII but not of AtHsp17.6-CII or LpHsp17.4-CII.

HsfA2 Controls the Activity of Developmentally and Stress-Regulated Heat Stress Protection Mechanisms in Tomato Male Reproductive Tissues

“…In this sense, the stability and activity of HsfA2 is controlled by interacting with HsfA1a and class CI and CII sHsps (Port et al, 2004), thereby HsfA2 becomes the major Hsf involved in regulation of acquired thermotolerance (Charng et al, 2007). Here, we show that the knockdown of tomato HsfA2 expression leads to significant reduction of the capacity of seedlings to acquire increased thermotolerance, i.e.…”

“…Consequently, HsfA2 becomes the most abundant Hsf during recovery or after several cycles of repeated HS (Hahn et al, 2011;Scharf et al, 1998). During recovery from HS, the availability of HsfA2 is controlled at the protein level by interactions with class CI and CII small Hsps (sHsps) and the activity of higher molecular weight chaperones such as Hsp70 and Hsp101 (Port et al, 2004). HsfA2 forms hetero-oligomeric complexes with HsfA1a.…”

A membrane-tethered transcription factor defines a branch of the heat stress response in Arabidopsis thaliana

“…Subsequent to the heat stress response, the activity of HSFs is attenuated and HSFs redistribute to the cytosol. In plants, HSBP1, HSP70, and HSP17.4-CII contribute to this down-regulation of HSF activity (31)(32)(33)(34). Membrane tethering of bZIP28 and the formation of HSF-HSP complexes would appear to be distinct mechanisms that allow plants to respond rapidly to heat stress that is localized in the ER and cytosol, respectively.…”

Role of Heat Stress Transcription Factor HsfA5 as Specific Repressor of HsfA4

“…The generation and use of specific antisera against individual tomato (Lycopersicon esculentum) Hsfs (HsfA1, A2, A3) and Hsp17-CI were described before (12,21,29). Primary antibodies for immunodetection of green fluorescent protein (GFP)-, Myc-, HA-, and Strep-tagged proteins were obtained from Roche Diagnostics (Mannheim, Germany), HISS Diagnostics (Freiburg, Germany), and IBA (Göttingen, Germany).…”

“…Protein extraction, SDS-PAGE, and protein blotting analysis were performed as described (19,29). The generation and use of specific antisera against individual tomato (Lycopersicon esculentum) Hsfs (HsfA1, A2, A3) and Hsp17-CI were described before (12,21,29).…”