Abstract
Heat shock proteins 70 (Hsp70s) belong to the key molecular chaperones. They are expressed constitutively to keep the nascent protein in appropriate form before getting the native conformation or during transport to the relevant compartment.
The expression of Hsp70s is induced by elevated temperature due to the capacity to refold partially denaturated or denaturated proteins. Hsp70s could also assist in elimination of damaged proteins by proteolysis using ubiquitin-proteasome system.
Furthermore, other types of stress such as cold, drought, heavy metals, free radicals, and UV light are associated with synthesis of Hsp70s. A number of recently published studies show that just in condition of stress Hsp70s can acquire specific functions.
For example, Hsp70s participate in photosynthesis; the chloroplastic Hsp70 is involved in protection of PS II against photoinhibition, in the assembly of new PS II complex, and de novo synthesis of D1, crucial protein of PS II. In some model systems plant-stressor expression of Hsp70 was suggested as key factor in induction of plant tolerance.
Hsp70 is also involved in stabilization of metabolically important enzymes and in re-establishing of protein homeostasis. Hsp70 together with Hsp90 are involved in ABA signalling pathways, therefore can affect stomata closure.
The expression of Hsp70s is regulated by heat shock factors (HSF) that function as transcription factors or through pathways that lead to up-regulation of Hsp70. Changes in the membrane fluidity, changes in Ca2+ levels cause activation of Ca2+ dependent protein kinases and MAP kinase cascades and up-regulation of HSF.
Moreover, the transcription of other thermotolerance and stress responsible gene is affected by HSFs. With regard to existence of multigene family of Hsp70, high number of HSF and complex network of metabolic pathways involved in these proteins the role of individual Hsp70 in the cell under stress conditions is still interesting topic, which is worth studying