STRESS RESPONSES - Cyanobacteria - page 657

heat shock proteins (Hsps). The Hsps help in the the folding of newly formed proteins and also help
in the refolding of denatured proteins. The Hsps were initially discovered in Drosophila melanogaster
larvae that were exposed to heat shock (Ritossa, 1962). Since then considerable wealth of information
has accumulated on the physiology, biochemistry and functional interactions of the various classes
of Hsps. These are thus designated as “molecular chaperones”, and are recognized as a ubiquitous
feature of model organisms undergoing experimental stress. The Hsps are classifi ed on the basis of
their molecular weight into chaperones having high and low molecular weight. These are designated
as Hsp followed by the molecular weight in eukaryotes such as Hsp100, Hsp90, Hsp70 and Hsp60
which are the major chaperones. Correspondingly in prokaryotes such as E . coli , ClpB (96 kDa) and
ClpA (84 kDa) belong to the Hsp100 family of chaperones. These are Clp proteases already described
under cold shock response. ClpB exists as a hexa or heptameric ring and helps in the solubilization of
protein aggregates. ClpA exists as a hexameric ring and helps in the unfolding of protein aggregates
for solubilization. Heat shock protein, HtpG protein (71kDa) of E . coli belongs to the Hsp90 family
and this protein exists as a dimer and helps in the prevention of protein aggregation. HtpG protein
has the ability to recognize non-native proteins and can help refolding of proteins in presence of
GroEL, GroES and ATP. Studies on htpG gene deletion or disrupted mutants of E . coli revealed the
non-essential nature of this gene under both non-stress and stressed conditions. DnaK protein
(69 kDa) belongs to the Hsp70 family and forms a substrate-binding channel and helps in co- and
post-translational protein folding in association with DnaJ and GrpE acting as co-chaperones. DnaK
binds to around 20% of newly synthesized polypeptides in E . coli , assisting in their folding and
structural maintenance (Hartl and Hayer-Hartl, 2002). It also regulates the activity of key proteins
involved in stress management and cell division (Parsell and Lindquist, 1993; McCarty and Walker,
1994; Tomoyasu et al ., 1998). The two co-chaperones, DnaJ and GrpE regulate the ATPase activity
of E . coli DnaK. DnaJ enhances the substrate-binding activity of DnaK by stimulating its ATPase
activity whereas GrpE is a nucleotide exchange factor, associating and dissociating with DnaK in
presence or absence of ATP, respectively (Liberek et al ., 1991; Harrison, 2003). GroEL protein (57
kDa) corresponds to Hsp60. Many of the Hsp60 chaperones are also known as chaperonins. The
term chaperonin was originally coined by Ellis (1990) to designate non-heat-induced Hsp60. GroEL
exists as a two heptameric rings and in association with GroES (chaperonin-10) forms a functional
complex that helps in post-translational protein-folding. One type of chaperonins, called chaperonin-
60 (Cpn60) is homologous to GroEL protein of E . coli . Both DnaK (Hsp70) and GroEL (Hsp60) have
been identifi ed and characterized from various organisms and major cell organelles. Although
Hsp70 proteins are synthesized under stress conditions but due to their constitutive expression
under normal growth conditions these are considered as cellular thermometer (Craig and Gross,
1991). Thus these constitute the ubiquitous group of molecular chaperones that bring about protein-
protein interactions and help in the folding and assembly of proteins. Single chain chaperonins
are FtsH (Hfl B, 71 kDa), DegP (HtrA, 48 kDa) and Lon (87 kDa). FtsH forms a multimeric ring and
helps in the folding and assembly of membrane proteins. HtrA exists as two hexameric rings and is
the chaperone expressed at low temperatures. Lon exists as a tetramer and facilitates the substrate
sequestration for proteolysis. Hsp33 (33 kDa) exists as a dimer and gives protection during oxidative
stress. Ibp A and Ibp B of E. coli correspond to α-Hsp with a molecular weight of 16 kDa. This forms
a sphere of 24 subunits that prevent protein aggregation. The heat inducibility of these chaperones
and chaperonins is in the order of α-Hsp>DnaK>ClpB, HtpG, GroEL>Hsp33, Lon>ClpA.
A number of explanations have been put forward for the synthesis of Hsps in the stressed
cells. According to the classical model, the accumulation of denatured proteins in the cytoplasm
constitutes the major trigger (Craig and Gross, 1991; Bukau, 1993). Another model takes into account