Voyaging around ClpB/Hsp100 proteins and plant heat tolerance

Abstract

Temperature is one of the key physical parameters that fine tunes plant growth and development. However, above the optimal range, it can negatively affect the physiology of plants. Supraoptimal temperature brings incongruity in cellular proteostasis resulting in the build-up of insoluble toxic protein aggregates. To prevent protein misfolding and aggregation, cells deploy different strategies including synthesis of heat shock proteins (Hsp) belonging to different families, like small Hsps (sHsps), Hsp40, Hsp60, Hsp70. Once these aggregates are formed, their dissolution and recovery of the functional proteins occurs by the action of Caseinolytic Protease B (ClpB)/Hsp100, which are evolutionarily conserved in bacteria, fungi and plants. ClpB function during heat stress (HS) is important and appears indispensable, as mutant bacteria, yeast as well as plants lacking ClpB protein fail to survive HS. Genetic expression of ClpB proteins is modulated both by high temperature as well as developmental cues. Plant contains three isoforms of ClpB/Hsp100, one each localized to cytoplasm (ClpB-C), chloroplast (ClpB-P) and mitochondria (ClpB-M), against one in bacteria and two in yeast. Among these, ClpB-C protein in particular governs the thermotolerance response in plants. This review introduces plant ClpB proteins, summarizes the knowledge gained hitherto in ClpB biology, critically analyzes the recent findings and brings forth the areas requiring thrust in the upcoming research on ClpBs.