Evolution, structure and function of the small heat shock proteins in plants

The a-crystallin-related, small heat shock proteins (smHSPs) are ubiquitous in nature, but are unusually abundant and diverse in higher plants as opposed to other eukaryotes. The smHSPs range in size from ~ 17 to 30 kDa and share a conserved C-terminal domain common to all eukaryotic smHSPs and to the a-crystallin proteins of the vertebrate eye lens. In higher plants six nuclear gene families encoding smHSPs have been defined. Each gene family encodes proteins found in a distinct cellular compartment, including the cytosol, chloroplast, ER, and mitochondrion. Evolutionary analysis suggests that the smHSP gene families arose by gene duplication and divergence prior to the radiation of angiosperms. In general, the smHSPs are not found in normal vegetative tissues, but accumulate to high levels in response to heat stress. Specific smHSPs are also expressed during various phases of plant development as part of the endogenous developmental programme. Thus, although the smHSPs are apparently not essential for basal cell functions as are the high molecular weight HSPs such as HSP90, HSP70 and HSP60, their functions are likely to be critical for survival and recovery from heat stress as well as for specific developmental processes. Biochemical analysis indicates that smHSPs are found in high molecular weight complexes between 200-400 kDa that are most likely composed solely of multiple smHSP subunits. Purified recombinant plant smHSPs facilitate reactivation of chemically denatured enzymes in a nucleotide-independent fashion and also prevent heat-induced aggregation or reverse inactivation of protein substrates. Based on these data, it is suggested that smHSPs act in vivo as a type of molecular chaperone to bind partially denatured proteins preventing irreversible protein inactivation and aggregation, and that smHSP chaperone activity contributes to the development of thermotolerance.