ABSTRACT
Exposure of organisms to a sudden increase in temperature triggers a ubiquitous and homeostatic cellular stress response. This heat shock response is characterized by rapid induction of genes encoding heat shock proteins (HSPs), which are divided into seven functional classes: metabolism, DNA/RNA repair, protein degradation, regulation, transport, cell organization, and molecular chaperones [1], and references therein. An almost identical functional classication of stress proteins was suggested by Kültz [2]. The predominant class of HSPs, the molecular chaperones [3], comprises ve major and evolutionarily conserved families-ClpB (in eukaryotes HSP100), HtpG (HSP90), dnaK (HSP70), GroESL also called as chaperonins (HSP60), and small heat shock proteins (sHSPs). Under physiological conditions, they assist in several important cellular processes including the correct folding of de novo synthesized proteins, the assembly of oligomeric structures, the membrane transport of proteins, etc. [4]. Upon heat stress, polypeptides tend to lose their native conformation, which leads to the increased exposure of hydrophobic amino acid sequences normally buried inside the molecule. Molecular chaperones are able to recognize these patches and
6.1 Introduction .......................................................................................................................... 145 6.2 Big Five: Function and Regulation of Molecular Chaperones in Synechocystis 6803 ......... 146
6.2.1 ClpB Family .............................................................................................................. 146 6.2.2 HtpG Family ............................................................................................................. 146 6.2.3 DnaK Family ............................................................................................................ 147 6.2.4 Chaperonins .............................................................................................................. 147 6.2.5 Small Heat Shock Proteins ....................................................................................... 148
6.3 Evolution of Membrane Sensor Hypothesis: Stress Sensors and Signaling in Synechocystis 6803 ........................................................................................................... 149
6.4 Additional Factors Involved in the Regulation of the Chaperone Genes of Synechocystis ... 150 Acknowledgment ........................................................................................................................... 151 References ...................................................................................................................................... 151
help to prevent their aggregation and facilitate the refolding of the damaged proteins [1,5]. Based on their activity, chaperones can be divided into three classes. “Foldases” (dnaK, GroEL) assist in the correct (re)-folding of polypeptides together with their co-chaperones. The “holdases” (sHSPs and HtpGs) are able to bind nonnative proteins and prevent their aggregation, but do not directly facilitate their folding. The third class of chaperones (“unfoldases/disaggregases,” ClpB family) is capable to dissolve aggregates [1,5,6].
