Molecular chaperones: Do they have a role in cold stress responses of plants?

Molecular chaperones are proteins that assist in the in vivo biogenesis of enzymes and structural proteins. They participate in biogenesis in several ways by: binding to nonnative nascent peptides emerging from ribosomes thereby preventing irreversible aggregation prior to folding, maintaining translocation across organelle membranes by stabilizing unfolded translocation competent forms, and helping in the assembly of oligomeric complexes. Numerous aspects of these processes are sensitive to high temperatures and consequently many molecular chaperones were first characterized as heat shock proteins. Generally lower temperatures increase the stability of proteins favoring the native state. However, there is a theoretical basis for a decreased stability and denaturation of some, so called "cold labile" proteins, and some aspects of translocation and assembly may also be similarly influenced by low temperature. The dehydration stress imposed during a freeze/thaw cycle may further alter the intracellular milieu in ways that could favor protein denaturation. An examination of the RNA levels of several members of one family of molecular chaperones, the HSP70s, in response to exposure of spinach to 5 degrees C revealed a pattern of differential expression that is consistent with a hypothesis that suggests that certain components of the protein biogenesis machinery requires some level of augmentation. It is proposed that chilling injury may arise, in part, from an impairment of normal protein biogenesis leading to an inability to form, or maintain, functional enzymes and structural proteins essential for cell homeostasis. Since the native state is stabilized at low temperature for most proteins, abnormalities in protein biogenesis would not be a global consequence, but only affect a subset of the proteins present in chilling sensitive plants.