Description
All organisms respond to high temperatures by synthesizing a group of proteins called Heat Shock Proteins (HSPs) that prevent unwanted protein aggregation during high temperature stresses. These molecular chaperones are ubiquitous, and are present in all three domains of life: archaea, bacteria, and eukarya. Small heat shock proteins (sHSPs) are a diverse family of HSPs crucial in plant stress responses as they have also been found to be expressed in response to a variety of other stresses including osmotic stress, oxidative stresses, high light and UV exposure. The sHSPs are highly diverse in both sequence and cellular location. Eleven different plant sHSP subfamilies have been identified based on their sequence evolution and their cellular localization. Although proteins belonging to the sHSP superfamily are diverse in sequence and size, sHSPs are characterized by the presence of a conserved _-crystallin domain of ~90 amino acid residues and have a molecular mass of 16-42kDa. The sHSPs have a conserved structural organization of 3 regions, a variable N-terminal region followed by the conserved _-crystallin domain and a short C-terminal region. Another group of proteins called ACDs have also been identified to share similarities in the _-crystallin region of the sHSPs and having a few differences in some of the conserved regions. The evolutionary relationships of sHSPs among different species utilizing the complete genome sequence data available for 20 plant species will enable us to determine the evolutionary history of this important group of proteins and at the same time make a distinction between sHSPs and the ACD group of proteins. The aim of this study is to come up with criteria to distinguish between the two groups of proteins based on sequence variations and functional relationships. The gene expression analysis of the sHSP and ACD probesets in Arabidopsis thaliana identified the differences in the expression patterns and functions of the two groups of proteins for various stress experiments. Despite the large levels of sequence diversity in sHSPs between different species and among subfamilies, substantial sequence conservation was identified. This study revealed the differences in the sequence organization, structures and gene expression profiles of the sHSP and ACD group of proteins
