STRESS RESPONSES - Cyanobacteria - page 666

C-terminus (Bienert et al ., 2006). Karradt et al . (2008) showed that the NblA protein interacts with the
ClpC from Anabaena sp. strain PCC 7120 with its highly conserved N-terminal helix to bring about
the degradation. The gene encoding the Clp protein, identifi ed as alr2999 , initially was annotated as
an “endopeptidase Clp ATP-binding chain ClpB” (bacteria.kazusa.or.jp/cyanobase/) but the BLAST
search against the NCBI database (www.ncbi.nlm.nih.gov/BLAST/) revealed it to be identical (87%)
to ClpC from Synechocystis sp. strain PCC 6803. Based on the site-directed mutagenesis at the highly
conserved N-terminal domain and the C-terminal domain, the interactions of NblA protein with
ClpC Alr2999 protease and phycobiliproteins, respectively have been confi rmed.
b) Deg Proteases : This family of ATP-dependent serine endopeptidases, wide-spread in bacteria
and eukaryotes, is responsible for the degradation of misfolded and abnormal proteins of the cell
membrane or periplasmic compartment (Strauch and Beckwith, 1988; Lipinska et al ., 1990; Kim et
al ., 1999). This family is also known as Htr family of proteases and includes three related proteins
DegP (HtrA), DegQ (HhoA) and Deg S (HhoB). These are very fl exible with regard to their substrate
specifi city. Some of these cleave only one substrate but others act as general proteases on unfolded
substrates. The proteolytic activity of these proteases is regulated by the PDZ domain [PDZ domain
represents a common structural domain of 80-90 amino acids found generally in the signalling
proteins of bacteria, yeasts, plants, viruses and animals. PDZ combines the fi rst letters of three such
proteins which were discovered for the fi rst time, i.e. post-synaptic density protein, Drosophila disc
large tumor suppressor (DlgA) and Zonula occludens-1 protein]. PDZ is also referred as DHR (Dlg
homologous region) or GLGF (Glycine-Leucine-Glycine-Phenylalanine) domains. DegP of E . coli
is a heat stress protease that functions in a homohexameric state in the periplasm (Kim et al ., 1999;
Sassoon et al ., 1999) and is required for cell viability at elevated temperatures or under oxidative stress
(Strauch et al ., 1989; Lipinska et al ., 1990; Skorko-Glonek et al ., 1997, 1999). At low temperatures DegP
functions as a molecular chaperone while at high temperatures it develops proteolytic activity (Spiess
et al ., 1999). Crystal structure of DegS of E . coli revealed that the PDZ domain is extended sideways
to form a funnel shaped structure and the proteolytic centre existed in an inactive conformation.
Specifi c interactions leading to a conformational shift and the activation of the proteolytic centre
are required (Wilken et al ., 2004). In contrast, DegP of E . coli consists of two PDZ domains, PDZ1
and PDZ2. A cage-like hexameric complex is produced by the assembly of two trimers (Clausen et
al ., 2002; Krojer et al ., 2002).
The genome of Synechocystis sp. strain PCC 6803 revealed the presence of three genes encoding
HtrA ( htrA , slr1204 ), HhoA ( hhoA , sll1679 ) and HhoB ( hhoB , sll1427 ) (Kaneko et al ., 1996). Though
designated similar to Deg proteases of E . coli , these appear to be more similar to each other than to
E . coli Deg proteases in their structure and function (Kieselbach and Funk, 2003; Huesgen et al ., 2005;
Jansen et al ., 2005). HhoA is a soluble protein located in the periplasm and its association with plasma
membrane has been demonstrated (Fulda et al ., 2000; Huang et al ., 2006). The role of Deg proteases
in protecting the cells from high light-induced damage was examined in a glucose tolerant strain
of Synechocystis . A comparison of a triple mutant (involving the step-wise inactivation of hhoA , then
hhoB double mutant and fi nally htrA ) with wild-type revealed that the amount of D1 in the mutant
was estimated to be approximately 60% of the original level. But the amounts of FtsH protease in
the wild-type and mutant were similar as found through immunodetection assy. These observations
led them to conclude that these Deg proteases are involved in the resistance of Synechocystis to light
stress and play either a direct or indirect role in the repair of PSII in vivo (Silva et al ., 2002). Since
the triple mutant did not survive at elevated growth temperatures or high light intensities (Silva
et al ., 2002; Barker et al ., 2006), it was concluded that that the Deg proteases protect Synechocystis