Novel Form of ClpB/HSP100 Protein in the Cyanobacterium Synechococcus

Abstract

Synechococcus sp. strain PCC 7942 has a second clpB gene that encodes a 97-kDa protein with novel features. ClpBII is the first ClpB not induced by heat shock or other stresses; it is instead an essential, constitutive protein. ClpBII is unable to complement ClpBI function for acquired thermotolerance. No truncated ClpBII version is normally produced, unlike other bacterial forms, while ectopic synthesis of a putative truncated ClpBII dramatically decreased cell viability.In response to rising growth temperatures, all organisms synthesize heat shock proteins (HSPs). Many of these inducible polypeptides are members of larger families based on similar size and sequence homology, and most include proteins synthesized constitutively under nonstress conditions. Members of most HSP families function as molecular chaperones in assisting the folding, assembly, and translocation of other proteins during normal and adverse growth regimens. One relatively new family of chaperones is HSP100/Clp. It can be divided into two basic groups, with proteins in the first (ClpA to -E) having two distinct ATP-binding domains (ATP-1 and ATP-2) while proteins in the second (ClpX and -Y) possess only one such domain (21). The main HSP of this chaperone family is ClpB, and most organisms produce at least two different types. Separate nuclear clpB genes in eukaryotes encode mitochondrial (78-kDa [11]) and cytosolic (100-to 110-kDa [19]) proteins, while plants have another ClpB isomer localized in chloroplasts (10). In contrast, a single gene in eubacteria encodes two differently sized proteins (ca. 79 and 93 kDa) via dual translational initiation sites within the clpB transcript (5, 23).ClpB is essential for resistance to high-temperature stress. The cytosolic ClpB in Saccharomyces cerevisiae and plants confers acquired thermotolerance (9,18,19), which is commonly defined as the resistance developed by an organism to withstand an otherwise lethal temperature treatment by being preexposed to a nonlethal high temperature. Similarly, the heat shock-inducible ClpBI protein from the cyanobacterium Synechococcus sp. strain PCC 7942 (Synechococcus) is also crucial for thermotolerance (5), a role that can be complemented by Escherichia coli ClpB (6). In the case of ClpBI, both the fulllength and truncated forms of the protein contribute to the thermotolerance acquired by Synechococcus (3).Mechanistically, ClpB functions to dissolve inactive protein aggregates that accumulate at high temperatures (16). In both bacteria and eukaryotes, ClpB cooperates with the DnaKDnaJ-GrpE proteins in a bichaperone network to prevent and revert protein aggregates during heat shock (7,13,14). ClpB apparently dissolves large, stable heat-inactivated proteins by directly binding to aggregates and exposing hydrophobic surfaces within the polypeptides by ATP-induced structural changes in the ClpB protein (4, 8).All ClpB proteins studied to date are strongly induced by high temperatures, and most are vital for heat tolerance. Under nonstressed conditions, however, ClpB i...

Effects of high light on transcripts of stress-associated genes for the cyanobacteria Synechocystis sp. PCC 6803 and Prochlorococcus MED4 and MIT9313

“…The three cyanobacteria studied here have two different clpB genes, as in Synechococcus sp. PCC 7942 (Eriksson et al, 2001). In the present study, clpB1 transcripts transiently increased while clpB2 mRNA levels did not change following the transfer to HL (Tables 3-5).…”

“…ClpB1 was shown to be essential for resistance to highand low-temperature stress in Synechococcus elongatus Porankiewicz & Clarke, 1997). While ClpB2 has been suggested to be an essential constitutive protein with general chaperone activity (Eriksson et al, 2001), our data indicate that ClpB1 might have a stress-related function. The clpP transcripts also accumulated in Synechocystis sp.…”

“…Furthermore, while transcripts encoding ClpC (Kaneko et al, 1996; declined following HL exposure of Synechocystis sp. PCC 6803 (Eriksson et al, 2001) and Prochlorococcus MIT9313, they showed a sustained increase in Prochlorococcus MED4. Prochlorococcus MED4 is found in ocean surface waters, where light intensities can reach up to 1500 mmol photons m 22 s 21 at midday, nutrient levels can be extremely low (Partensky et al, 1999), and the cells probably experience oxidative stress.…”

“…A number of these genes may be induced when cells are shifted to high temperatures (Eriksson et al, 2001) or from LL to HL. In Synechocystis sp.…”

ClpB1 Overproduction in Synechocystis sp. Strain PCC 6803 Increases Tolerance to Rapid Heat Shock

“…The production of heat shock proteins (HSPs) is one of the most ubiquitous stress-defense mechanisms (8), mainly studied as a primary response to rapidly rising temperatures. One of the first genes to be induced after heat shock is clpB, with its gene product possibly serving a dual function (9): disaggregation of large protein complexes (10) and aiding DnaK2 with resolubilization of damaged proteins (11). The active form of ClpB is a hexamer that is predicted to be soluble in the cytoplasm (12).…”