Interaction of GroE with an all-beta-protein.

Molecular chaperones are involved in protein folding both in vivo and in vitro. The Escherichia coli chaperone GroEL interacts with a number of nonnative proteins. A common structural motif of nonnative proteins, which is recognized by GroEL, has not yet been identified. In order to study the role of beta-sheet secondary structure on the interaction of nonnative proteins with GroEL, we used the F(ab) fragment of a monoclonal antibody as a model substrate protein. Here we show that GroEL interacts functionally with this all-beta-protein during reactivation. Antibody fragments refold spontaneously in good yield from the guanidine-denatured state. Functional refolding to the native state is inhibited transiently by GroEL, but there is no complete folding arrest in the absence of Mg-ATP and GroES. The yield of these unspecifically released GroEL-bound F(ab) fragments corresponds to that of the spontaneous reactivation in the absence of chaperones. However, the refolding kinetics in the presence of GroEL are considerably slower. The addition of Mg-ATP to the GroEL.F(ab) complex results in an immediate release of bound substrate protein and a significant increase in the amount of reconstituted antibody fragments compared to spontaneous reactivation. GroES is not essential for functional GroEL-mediated refolding of the F(ab) fragment but affects the reactivation yield to a small extent. Interestingly, stimulation of the GroEL-mediated F(ab) refolding depends primarily on the binding and not on hydrolysis of adenosine triphosphates. Previous results indicate the binding of alpha-helices to GroEL. The results presented in this paper suggest that beta-sheet secondary structural elements are recognized by GroEL. We therefore conclude that the interaction of a nonnative protein with GroEL depends mainly on the nature of the early folding intermediate but not on a specific element of secondary structure.     

The heat shock response: life on the verge of death

Organisms must survive a variety of stressful conditions, including sudden temperature increases that damage important cellular structures and interfere with essential functions. In response to heat stress, cells activate an ancient signaling pathway leading to the transient expression of heat shock or heat stress proteins (Hsps). Hsps exhibit sophisticated protection mechanisms, and the most conserved Hsps are molecular chaperones that prevent the formation of nonspecific protein aggregates and assist proteins in the acquisition of their native structures. In this Review, we summarize the concepts of the protective Hsp network.     

The activation mechanism of Hsp26 does not require dissociation of the oligomer

Small heat shock proteins (sHsps) are molecular chaperones that specifically bind non-native proteins and prevent them from irreversible aggregation. A key trait of sHsps is their existence as dynamic oligomers. Hsp26 from Saccharomyces cerevisiae assembles into a 24mer, which becomes activated under heat shock conditions and forms large, stable substrate complexes. This activation coincides with the destabilization of the oligomer and the appearance of dimers. This and results from other groups led to the generally accepted notion that dissociation might be a requirement for the chaperone mechanism of sHsps. To understand the chaperone mechanism of sHsps it is crucial to analyze the relationship between chaperone activity and stability of the oligomer. We generated an Hsp26 variant, in which a serine residue of the N-terminal domain was replaced by cysteine. This allowed us to covalently crosslink neighboring subunits by disulfide bonds. We show that under reducing conditions the structure and function of this variant are indistinguishable from that of the wild-type protein. However, when the cysteine residues are oxidized, the dissociation into dimers at higher temperatures is no longer observed, yet the chaperone activity remains unaffected. Furthermore, we show that the exchange of subunits between Hsp26 oligomers is significantly slower than substrate aggregation and even inhibited in the presence of disulfide bonds. This demonstrates that the rearrangements necessary for shifting Hsp26 from a low to a high affinity state for binding non-native proteins occur without dissolving the oligomer.     

Leaf rosette closure in the alpine rock species Saxifraga paniculata Mill.: significance for survival of drought and heat under high irradiation

The significance of leaf rosette closure for survival of drought and heat under high irradiation on alpine rock sites was investigated in the cushion forming rosette plant, Saxifraga paniculata Mill. With decreasing water content the leaves fold over the rosette centre reducing reversibly the evaporative leaf surface area by 80%. Internal water redistribution driven by an osmotic gradient from older to younger leaves occurs. The oldest leaves dry out to promote the survival of the individual. Leaf temperatures above 45 °C (which match heat tolerance limits 45–57 °C; LT₅₀) co-occurred with low substrate water potentials (less than – 0·5 MPa) on 11·3% of summer days. Shading by leaves can be crucial to surviving high temperatures as it keeps the rosette centre up to 10 °C colder. Mutual shading prevented sustained drought-induced photoinhibition in upper leaf surfaces at relative water contents below 60%. In exposed lower leaf surfaces restoration of photosystem II took several days. Leaf temperatures above 40 °C (21·3% of summer days) induced photoinhibition in situ. Periods with sufficient water supply can be fully utilized as rehydration is fast ( < 12 h) and exposes the upper leaf surfaces that showed only minor photoinhibition. By reversible leaf rosette closure environmental extremes that otherwise could exceed tolerance are efficiently avoided.     

Folding mechanism of the CH2 antibody domain

The immunoglobulin C(H)2 domain is a simple model system suitable for the study of the folding of all-beta-proteins. Its structure consists of two beta-sheets forming a greek-key beta-barrel, which is stabilized by an internal disulfide bridge located in the hydrophobic core. Crystal structures of various antibodies suggest that the C(H)2 domains of the two heavy chains interact with their sugar moieties and form a homodimer. Here, we show that the isolated, unglycosylated C(H)2 domain is a monomeric protein. Equilibrium unfolding was a two-state process, and the conformational stability is remarkably low compared to other antibody domains. Folding kinetics of C(H)2 were found to consist of several phases. The reactions could be mapped to three parallel pathways, two of which are generated by prolyl isomerizations in the unfolded state. The slowest folding reaction, which was observed only after long-term denaturation, could be catalyzed by a prolyl isomerase. The majority of the unfolded molecules, however, folded more rapidly, on a time-scale of minutes. Presumably, these molecules also have to undergo prolyl isomerization before reaching the native state. In addition, we detected a small number of fast-folding molecules in which all proline residues appear to be in the correct conformation. On both prolyl isomerization limited pathways, the formation of partly structured intermediates could be observed.     

Sti1 is a novel activator of the Ssa proteins

The molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins in eukaryotes. Of specific importance in this context is a ternary multichaperone complex in which Hsp70 and Hsp90 are connected by Hop. In Saccharomyces cerevisiae two components of the complex, yeast Hsp90 (yHsp90) and Sti1, the yeast homologue of Hop, had already been identified, but it remained to be shown which of the 14 different yeast Hsp70s are part of the Sti1 complex and what were the functional consequences resulting from this interaction. With a two-hybrid approach and co-immunoprecipitations, we show here that Sti1 specifically interacts with the Ssa group of the cytosolic yeast Hsp70 proteins. Using purified components, we reconstituted the dimeric Ssa1-Sti1 complex and the ternary Ssa1-Sti1-yHsp90 complex in vitro. The dissociation constant between Sti1 and Ssa1 was determined to be 2 orders of magnitude weaker than the affinity of Sti1 for yHsp90. Surprisingly, binding of Sti1 activates the ATPase of Ssa1 by a factor of about 200, which is in contrast to the behavior of Hop in the mammalian Hsp70 system. Analysis of the underlying activation mechanism revealed that ATP hydrolysis is rate-limiting in the Ssa1 ATPase cycle and that this step is accelerated by Sti1. Thus, Sti1 is a potent novel effector for the Hsp70 ATPase.