Heat Shock Proteins

During evolution, plants have developed sophisticated mechanisms to sense the subtle changes of gr owth conditions, and trigger signal transduction cascades, which in him activate stress responsive genes and ultimately lead to changes at the physiological and biochemical levels.Abiotic stress especially thermal stress adversely affects the functioning of cellular and metabolic pathways in plants. One of the main effects is on functioning of normal cellular proteins. Under thermal stress there is aggregation and misfolding of important cellular proteins occurred. Plants have developed different defense mechanisms to adapt with these adverse conditions. Under the course of defense mechanism at molecular level, transcription and translation of special set of proteins like Heat Shock Proteins (HSPs) occurs (Kotak et ah, 2007, Kumar et al., 2016). Diversification in HSPs may reflect an adaptation to tolerate the heat stress. These molecular chaperones assist in protein refolding under stress conditions, protects plants against stress by re-establishing normal protein conformation and thus cellular homeostasis.

Under stressful condition, cell response triggered the production of heat shock proteins (HSP). They were named heat shock protein as fust described in relation to heat shock, but are now also known to be expressed during other stresses like exposure to cold, UY light, during wound healing or tissue remodeling. Many HSPs also functions as chaperone by stabilizing new proteins or by helping the refolding of damaged proteins of the cell due to stress (Figure 1). This increase in the expression of HSPs are transcriptionally regulated and the dramatic upregulation of the heat shock proteins is a key heat shock response and is induced primarily by heat shock factors (Hsfs) that are located in the cytoplasm in an inactive state. These factors are considered as transcriptional activators for heat shock (Baniw^ral et ah, 2004; Hu et al., 2009). HSPs are found in virtually all living organisms, from bacteria to plants and humans.

Fig. 1: Schematic representation of functional overview of HSP

Thermal Stability of HSPs

Incorrect protein folding into cells can cause several conformational disorders and in order to prevent such structural misfolding and to maintain homeostasis, cells have evolved an efficient protein quality control system (PQC) as an endogenous process. This PQC system needed molecular chaperones (including all HSP families) and then main function is to prevent inappropriate interactions, avoiding protein aggregation by assisting then correct folding and if protein collection is not possible, guiding them to cell degradation system. To maintain the thermal stability of proteins, the chaperone system changes from a folding to a storing function at heat shock temperatures. The temperature at which this change occurs depends on the presence of a thennosensor in at least one of the components of the chaperone systems. One of the most important chaperones is the Heat-shock protein 90 kDa (HSP90), which is responsible for the collect folding of a wide range of proteins. In the folding process, it is essential that HSP90 form complexes with co-chaperones, and providing a cooperative action during the maturation cycle of client proteins.

Classification of Heat Shock Proteins

Historically, the discovering of the heat-shock proteins was started, when the Italian Scientist F. Ritossa observed the expression of gene in the chromosomes puffing of Drosophila melanogaster after exposure to heat shock. Increase in protein synthesis was observed that occurred also by the use of other stress factors such as azide, salicylate and 2,4-dinitrophenol (Ritossa, 1962). After that report, these proteins were identified and named as heat-shock protein (HSP) (Tissieresef al, 1974). Thereafter, various studies were started to find out the relationship of the synthesis of these proteins with the tolerance of stresses. On the other hand, Lin

et al, (1984) reported that the exposure of Glycine max seedlings to heat shock (from 28 to 45 °C) for 10 min (longer periods killed the seedlings) induce the synthesis of HSPs at the cost of other proteins synthesis.

Several types of heat shock proteins have been identified in almost all organisms (Bliarti and Nover, 2002). HSPs are mainly characterized on the basis of the presence of a carboxylic terminal called heat-shock domain (Helm et ah, 1993). HSPs having molecular weights ranges from 10 to 200 kDa are characterized as chaperones where they participate in the induction of the signal during heat stress (Schoffl et al., 1998). Heat-shock proteins of archaea have been classified on the basis of their approximate molecular weight as: (1) Heat shock protein of molecular weight 100 kDa: HSP 100, (2) HSP90, (3) HSP70, (4) HSP60, and small heat- shock proteins (sHSPs) where the molecular weight ranges from 15 to 42 kDa (Trent, 1996). Schlesinger (1990) reported that in eukaryotic organisms, the principle heat-shock proteins of human beings do not differ from those of bacteria except for the presence of HSP33. Later, the HSPs of human beings were grouped into five families (Kregel, 2002) as in Table 1.

In plants, according to molecular weight,amino acid sequence homologies and functions, five classes of HSPs are characterized: (1) HSP 100, (2) HSP 90, (3) HSP 70, (4) HSP 60 (5) small heat-shock proteins (sHSPs) (Kotak et al, 2007,Gupta et al, 2010).

Table 1: Families of HSPs in human beings, their site, and suggested functions (Kregel, 2002)

The high molecular weight HSPs are characterized as molecular chaperone. Higher plants have at least 20 sHSPs and there might be 40 kinds of these sHSPs in one plant species.

The name of HSPs in bacteria differ from those in eukaiyotic cells as given below but the nomenclature for sHSPs are same in both the organisms (Kotak et «/.,2007).