STRESS RESPONSES - Cyanobacteria - page 665

However, a gradual and slow accumulation of the respective proteins during high light stress and
low temperature (except ClpP2 protein) occurred. Despite the occurrence of ClpPII and ClpPIII with
ClpX and ClpR in bicistronic operons, the transcripts of these genes were produced as monocistronic
constructs. Mutants defective in ClpPI produced lower levels of ClpP2 protein concomitant with
increased levels of both ClpP3 and ClpR. The inactivation of ClpPII gene caused a rise in ClpPI
transcripts but not the ClpP1 protein level. These results point out to the functional nature of the
ClpP genes and there exists a regulatory complexity among these genes (Schelin et al ., 2002).
Andersson et al . (2006) described the chaperone activities of ClpC from S . elongatus PCC 7942
(SyClpC) overexpressed in E . coli. Such purifi ed SyClpC exhibited in vitro chaperone activity both by
preventing aggregation of unfolded polypeptides and by refolding the aggregated proteins into their
native state. Gel fi ltration chromatography analysis revealed SyClpC to be a dimer but in presence
of ATP and casein it forms a hexamer similar to other Hsp100 proteins. ATPase activity of SyClpC
was found to be optimal between 37°C and 45°C within a pH range of 7.0 to 8.0. The presence of
an adaptor protein MecA from B . subtilis enhanced the refolding activity of SyClpC by 3-fold. Since
cyanobacteria lack MecA orthologs, the role of ClpS adaptor proteins (ClpS1 and ClpS2) in enhancing
the ATPase activity of SyClpC was tested but SyClpS1 did not stimulate the ATPase activity of
SyClpC. But it may be worthwile to mention that ClpS adaptor proteins bind directly to N-terminal
destabilizing residues on the substrate protein, targeting them for degradation by ClpAP (Dougan
et al ., 2002; Erbse et al ., 2006). Stanne et al . (2007) reported the existence of two mixed proteolytic
cores. The fi rst one consisted of ClpP1 and ClpP2 subunits associated with ClpX while the second
one contained ClpP3 and ClpR that were bound to ClpC. Of these two, the latter was shown to be
constitutively abundant and was required for cell viability and growth (Clarke and Eriksson, 1996;
Schelin et al ., 2002). The localization of all Clp proteins in S. elongatus PCC 7942 was found to be
in the soluble fraction except that a minor proportion (20-40%) of ClpC, ClpP1 and ClpR was also
associated with membrane fraction. The levels of the Clp proteins in mutants defective in ClpPI and
ClpPII were compared with the wild-type in relation to their localization. Consistent with the earlier
observations of Schelin et al . (2002) mutant defective in ClpP1 showed a down-regulation of ClpP2
with a simultaneous increase in ClpP3 and ClpR. The localization of the latter protein increased in
the membrane fraction in ClpP1 mutant when compared to wild-type although the former protein
was associated in traces with the membrane fraction. The levels of ClpX were higher in the ClpP1-
defi cient mutant without any apparent change in the levels of ClpC, ClpS1 and ClpS2 relative to
wild-type. In ClpP2-defective mutant except that the ClpP1 was associated with membranes in
the absence of soluble ClpP2 protein, the levels of rest of the proteins resembled those of ClpP1-
defective muant. Andersson et al . (2009) purifi ed the ClpP3/ClpR complex and found it to consist
of a heptameric complex with three and four units of ClpP3 and ClpR, respectively. This complex
exhibited remarkable similarities to the eukaryotic 20 S proteosome.
Nitrogen straved cells of cyanobacteria generally exhibit chlorosis due to the degradation of
phycobiliproteins. Although the detailed mechanism of degradation is not yet known, the role of
“non-bleaching genes” has been identifi ed after screening a number of non-bleaching ( nbl ) mutants of
S. elongatus PCC 7942. Of these genes, nblA encodes a protein NblA that has been directly implicated
in the degradative process. Crystal structure of NblA polypeptide from Anabaena sp. strain PCC 7120
revealed the existence of two α-helices, a shorter N-terminal and a longer C-terminal one. Through
their C-terminal domains, the NblA molecules undergo dimerization leading to a homodimer
formation (Bienert et al ., 2006). This has been confi rmed from the crystallographic data from the NblA
proteins of Thermosynecococcus vulcanus as well (Dines et al ., 2007). The interaction of NblA with the
α-subunits of phycobiliproteins is mediated through the highly conserved amino acid residues at the