The evolution, function, structure, and expression of the plant sHSPs.

Small heat shock proteins are a diverse, ancient, and important family of proteins. All organisms possess small heat shock proteins (sHSPs), indicating that these proteins evolved very early in the history of life prior to the divergence of the three domains of life (Archaea, Bacteria, and Eukarya). Comparing the structures of sHSPs from diverse organisms across these three domains reveals that despite considerable amino acid divergence, many structural features are conserved. Comparisons of the sHSPs from diverse organisms reveal conserved structural features including an oligomeric form with a β-sandwich that forms a hollow ball. This conservation occurs despite significant divergence in primary sequences. It is well established that sHSPs are molecular chaperones that prevent misfolding and irreversible aggregation of their client proteins. Most notably, the sHSPs are extremely diverse and variable in plants. Some plants have >30 individual sHSPs. Land plants, unlike other groups, possess distinct sHSP subfamilies. Most are highly up-regulated in response to heat and other stressors. Others are selectively expressed in seeds and pollen, and a few are constitutively expressed. As a family, sHSPs have a clear role in thermotolerance, but attributing specific effects to individual proteins has proved challenging. Considerable progress has been made during the last 15 years in understanding the sHSPs. However, answers to many important questions remain elusive, suggesting that the next 15 years will be at least equally rewarding.