STRESS RESPONSES - Cyanobacteria - page 670

for maximum induction for cells grown at 36°C but this decreased to 38-40°C for cells grown at 22°C.
It means there is no particular temperature set point for the activation of heat shock genes. Moreover,
unlike the induction of other Hsps, hsp17 was induced at temperatures 44°C and above for samples
grown at 36°C. A redox signalling pathway explains the diffusion rate of plastoquinone molecule
and other mobile electron carriers also determine the thylakoid integrity and in turn the acquisition
of thermotolerance (Huner et al ., 1996). In addition, the induction of groESL and cpn60 were shown to
be controlled by the redox state of the thylakoid membranes (Glatz et al ., 1997). Due to the absence of
heat shock response when cytoplasmic membranes were selectively hydrogenated, these investigators
concluded that the thylakoid membranes act as a cellular thermometer to transduce signals of heat
stress. Török et al . (2001) substantiated the role of sHsp from Synechocystis sp. strain PCC 6803 and
showed that Hsp17 plays a dual role in stabilizing heat-stressed membranes as well as transferring
unfolded proteins to DnaK/DnaJ/GrpE and GroEL/ES chaperone network for subsequent refolding.
The function of stabilizing the membranes depended on its preference for the liquid crystalline phase.
So depending on whether Hsp17 exists in association with thylakoid membranes or cytosolic fraction,
it may function as a membrane stabilizing factor or as a member of multi-chaperone network. These
roles of Hsp17 were confi rmed by studies on hsp17 - deletion mutant. The sHsps (α-crystallin and
Hsp17) from Synechocystis sp. strain PCC 6803 interacted with the polar head group of synthetic
anionic membranes of dimyristoylphosphatidylglycerol and dimyristoylphosphatidylserine and
strongly stabilized the liquid-crystalline state. Likewise, these two sHsps prevented the formation
of inverted hexagonal structure and stabilized the bilayer liquid-crystalline state in membranes
composed of nonbilayer lipid didaidophosphatidylethanolamine. Membranes consisting of MGDG
and PG (enriched with unsaturated fatty acids and isolated from Synechocystis thylakoids) exhibited
an increased molecular order in the fl uid-like state after interaction with the sHsps. These results
suggest that the sHsps can modulate membrane lipid polymorphism (Tsvetkova et al ., 2002). Giese
and Vierling (2002) reported a deletion mutant (hsp16.6 - ) of Synechocystis sp. strain PCC 6803 that
showed a conditional lethal phenotype. The oligomeric nature (approximately 20 subunits) and
its potential to bring about refolding of bound denatured proteins in association with DnaK/DnaJ
chaperone system has also been reported (Mogk et al ., 2003). The role of sHsps in protecting wide
range of cellular functions of Synechocystis sp. strain PCC 6803 during heat stress has come to light by
employing immunoprecipitation and affi nity chromatography. As many as 42 different proteins have
been recovered which exhibited interactions with Hsp16.6 of Synechocystis . These were released by
the ATP-dependent activity of DnaK and co-chaperones and all of them were found to be heat labile.
Some of the proteins (13) identifi ed by mass spectrometry related to metabolic processes ranging
from transcription, translation, cell signalling and secondary metabolism. A comparative study of
the wild-type and sHsp 16.6 deletion mutant showed that sHsp 16.6 contributed signifi cantly to the
acquisition of cellular thermotolerance (Basha et al ., 2004). The dynamic nature of the interactions
between the subunits of sHsp oligomers has been investigated in a number of systems like wheat
(Hsp 16.9), yeast (Hsp28), Synechocystis (Hsp16.6) and Methanococcus tuberculosis (Hsp16.3) and their
reversible dissociation into subunits upon heating was observed (Haslbeck et al ., 1999; van Montfort
et al ., 2001; Giese and Vierling, 2002; Gu et al ., 2002). In order to understand the interactions of the
subunits of Synechocystis sp. strain PCC 6803 sHsp16.6 oligomers, Friedrich et al . (2004) designed the
sHsp16.6 gene constructs with a C-terminal 8 amino acid tags (WSHPQFEK represnting Trp-Ser-His-
Pro-Gln-Phe-Glu-Leu, respectively). E . coli (BL21) cells were transformed with such pJC2016.6 plasmid
and the transformants produced the C-terminal tagged sHsp16.6. Separation of C-terminal tagged
sHsp 16.6 by streptactin (resin) column chromatography and purifi cation to >95% homogeneity