Evolutionary analysis of the small heat shock proteins in five complete algal genomes.

Small heat shock proteins (sHSPs) are chaperones that are crucial in the heat shock response but also have important nonstress roles within the cell. sHSPs are found in all three domains of life (Bacteria, Archaea, and Eukarya). These proteins are particularly diverse within land plants and the evolutionary origin of the land plant sHSP families is still an open question. Here we describe the identification of 17 small sHSPs from the complete genome sequences of five diverse algae: Chlamydomonas reinhardtii, Cyanidioschyzon merolae, Ostreococcus lucimarinus, Ostreococcus tauri, and Thalassiosira pseudonana. Our analysis indicates that the number and diversity of algal sHSPs are not correlated with adaptation to extreme conditions. While all of the algal sHSPs identified are members of this large and important superfamily, none of these sHSPs are members of the diverse land plant sHSP families. The evolutionary relationships among the algal sHSPs and homologues from bacteria and other eukaryotes are consistent with the hypothesis that the land plant chloroplast and mitochondrion sHSPs did not originate from the endosymbionts of the chloroplast and mitochondria. In addition the evolutionary history of the sHSPs is very different from that of the HSP70s. Finally, our analysis of the algal sHSPs sequences in light of the known sHSP crystal structures and functional data suggests that the sHSPs possess considerable structural and functional diversity.

The Arabidopsis ClpB/Hsp100 family of proteins: chaperones for stress and chloroplast development.

The Casein lytic proteinase/heat shock protein 100 (Clp/Hsp100) proteins are chaperones that act to remodel/disassemble protein complexes and/or aggregates using the energy of ATP. In plants, one of the best-studied proteins from this family is cytosolic ClpB1 (At1g74310), better known in Arabidopsis as AtHsp101, which is a heat shock protein required for acclimation to high temperatures. Three other ClpB homologues have been identified in the Arabidopsis genome (ClpB2, ClpB3 and ClpB4; At4g14670, At5g15450 and At2g25140). To define further the roles of these chaperones in plants we investigated their intracellular localization, evolutionary relationships, patterns of expression and the phenotypes of corresponding T-DNA insertion mutants. We first found that ClpB2 was misannotated; there is no functional ClpB/Hsp100 gene at this locus. By fusing the putative transit peptides of ClpB3 and ClpB4 with GFP, we showed that these proteins are targeted to the chloroplast and mitochondrion, respectively, and we therefore designated them as ClpB-p and ClpB-m. Phylogenetic analysis supports two major lineages of ClpB proteins in plants, an 'eukaryotic', cytosol/nuclear-localized group containing AtHsp101, and an organelle-localized lineage, containing both ClpB-p and ClpB-m. Although AtHsp101, ClpB-p and ClpB-m transcripts all accumulate dramatically at high temperatures, the T-DNA insertion mutants of ClpB-p and ClpB-m show no evidence of seedling heat stress phenotypes similar to those observed in AtHsp101 mutants. Strikingly, ClpB-p knockouts were seedling lethals, failing to accumulate chlorophyll or properly develop chloroplasts. Thus, in plants, the function of ClpB/Hsp100 proteins is not restricted to heat stress, but a specific member of the family provides housekeeping functions that are essential to chloroplast development.