Small heat shock proteins (sHsps) are molecular chaperones involved in maintaining protein homeostasis; they have also been implicated in protein folding diseases and in cancer. In this protein family, a conserved core domain, the so-called α-crystallin or Hsp20 domain, is flanked by highly variable, nonconserved sequences that are essential for chaperone function. Analysis of 8714 sHsps revealed a broad variation of primary sequences within the superfamily as well as phyla-dependent differences. Significant variations were found in the number of sHsps per genome, their amino acid composition, and the length distribution of the different sequence parts. Reconstruction of the evolutionary tree for the sHsp superfamily shows that the flanking regions fall into several subgroups, indicating that they were remodeled several times in parallel but independent of the evolution of the α-crystallin domain. The evolutionary history of sHsps is thus set apart from that of other protein families in that two exon boundary-independent strategies are combined: the evolution of the conserved α-crystallin domain and the independent evolution of the N- and C-terminal sequences. This scenario allows for increased variability in specific small parts of the protein and thus promotes functional and structural differentiation of sHsps, which is not reflected in the general evolutionary tree of species.

The small heat shock protein αB-crystallin is an oligomeric molecular chaperone that binds aggregation-prone proteins. As a component of the proteostasis system, it is associated with cataract, neurodegenerative diseases, and myopathies. The structural determinants for the regulation of its chaperone function are still largely elusive. Combining different experimental approaches, we show that phosphorylation-induced destabilization of intersubunit interactions mediated by the N-terminal domain (NTD) results in the remodeling of the oligomer ensemble with an increase in smaller, activated species, predominantly 12-mers and 6-mers. Their 3D structures determined by cryo-electron microscopy and biochemical analyses reveal that the NTD in these species gains flexibility and solvent accessibility. These modulated properties are accompanied by an increase in chaperone activity in vivo and in vitro and a more efficient cooperation with the heat shock protein 70 system in client folding. Thus, the modulation of the structural flexibility of the NTD, as described here for phosphorylation, appears to regulate the chaperone activity of αB-crystallin rendering the NTD a conformational sensor for nonnative proteins.olecular chaperones share the ability to bind nonnative, aggregation-prone polypeptides and assist their folding and assembly (1-3). Among these, the small heat shock protein (sHsp) αB-crystallin (also HspB5) is one of the major constituents of the vertebrate eye lens where it functions both as a chaperone and structural protein (4, 5). In nonlenticular tissues, αB-crystallin (αB) participates in sustaining cellular proteostasis. The involvement in neurodegenerative diseases (6, 7), multiple sclerosis (8), myopathies (9), as well as in cell cycle control, apoptosis, and cancer (10, 11) underlines its importance for cellular proteostasis.αB exhibits a tripartite organization (Fig. 1A) with a central α-crystallin domain (ACD) flanked by an N-terminal domain (NTD) and a short C-terminal extension (CTE) (12, 13). The ACD forms stable dimers (14-16) that further assemble into higher-order oligomers via interactions mediated by the NTD and CTE (17)(18)(19). αB forms dynamic populations of multimers with a variable number of subunits (20,21). Structural studies indicate that the variety of oligomeric states including a symmetric 24-mer (22) is created by addition of subunits to (or subtraction from) existing oligomers (17, 18). As for many other sHsps (23), the polydispersity of αB is coupled to spontaneous subunit exchange of yet undetermined units. αB quaternary dynamics was attributed to fluctuations of the intersubunit contacts mediated by the C-terminal IXI motif (24). However, given its involvement in oligomer formation, the NTD must also play a decisive role.In general, sHsps including αB recognize aggregation-prone, partially unfolded substrates (4, 25, 26) and keep them in a refolding-competent state (27, 28). The substrate binding sites of sHsps have not been defined yet. Recent studies suggest the involvement ...

Oxidant-mediated antibacterial response systems are broadly used to control bacterial proliferation. Hypochlorite (HOCl) is an important component of the innate immune system produced in neutrophils and specific epithelia. Its antimicrobial activity is due to damaging cellular macromolecules. Little is known about how bacteria escape HOCl-inflicted damage. Recently, the transcription factor YjiE was identified that specifically protects Escherichia coli from HOCl killing. According to its function, YjiE is now renamed HypT (hypochlorite-responsive transcription factor). Here we unravel that HypT is activated by methionine oxidation to methionine sulfoxide. Interestingly, so far only inactivation of cellular proteins by methionine oxidation has been reported. Mutational analysis revealed three methionines that are essential to confer HOCl resistance. Their simultaneous substitution by glutamine, mimicking the methionine sulfoxide state, increased the viability of E. coli cells upon HOCl stress. Triple glutamine substitution generates a constitutively active HypT that regulates target genes independently of HOCl stress and permanently down-regulates intracellular iron levels. Inactivation of HypT depends on the methionine sulfoxide reductases A/B. Thus, microbial protection mechanisms have evolved along the evolution of antimicrobial control systems, allowing bacteria to survive within the host environment.