High selectivity with low specificity: how SecB has solved the paradox of chaperone binding

SecYEG and SecA are the stoichiometric components of preprotein translocase

The transport of large preproteins across the Escherichia coli plasma membrane is catalyzed by preprotein translocase, comprised of the peripherally bound SecA subunit and an integrally bound heterotrimeric domain consisting of the SecY, SecE, and SecG subunits. We have now placed the secY, secE, and secG genes under the control of an arabinose-inducible promoter on a multicopy plasmid. Upon induction, all three of the proteins are strongly overexpressed and recovered in the plasma membrane fraction. These membranes show a strong enhancement of 1) translocation ATPase activity, 2) preprotein translocation, 3) capacity for SecA binding, and 4) formation of the membrane-inserted form of SecA. These data establish that SecY, SecE, and SecG constitute the integral membrane domain of preprotein translocase.

Diffusion-limited interaction between unfolded polypeptides and the Escherichia coli chaperone SecB

SecB is a chaperone dedicated to protein translocation in Escherichia coli. SecB binds to a subset of precursor proteins, and targets them in a translocation-competent state to the SecA subunit of the translocase. The nature and kinetics of the interaction of SecB with polypeptides were studied by spectroscopic techniques using the reduced form of bovine pancreatic trypsin inhibitor (BPTI) as a model substrate. Binding of SecB to BPTI resulted in an increase in the fluorescence of the surface-exposed tryptophan residue 36 of SecB. SecB reversibly binds BPTI in stoichiometric amounts. Labeling of BPTI with the fluorophore acrylodan allowed the analysis of the binding reaction at nanomolar concentrations. High-affinity binding (KD of 5.4 nM) of labeled BPTI to SecB resulted in a blue shift of the acrylodan emission maximum and an increase in the fluorescence quantum yield, suggesting that BPTI binds in an apolar environment. Stopped-flow acquisition of rate constants of complex formation between SecB and BPTI yielded a second-order binding rate constant of 5 x 10(9) M-1 s-1, and a dissociation rate constant of 48 s-1. These data demonstrate that in vitro, the association of SecB with polypeptide substrates is limited by the rate of collision. In vivo, SecB binding is selective, and predominantly occurs with nascent polypeptides. Since these chains are not expected to fold into stable structures, SecB association may be governed by "more or less" specific interactions and be limited by the rate of chain elongation rather than the rate of folding.

1H NMR spectroscopy reveals that mouse Hsp25 has a flexible C-terminal extension of 18 amino acids

The small heat-shock proteins (Hsps) exist as large aggregates and function by interacting and stabilising non-native proteins in a chaperone-like manner. Two-dimensional 1H NMR spectroscopy of mouse Hsp25 reveals that the last 18 amino acids have great flexibility with motion that is essentially independent of the domain core of the protein. The lens protein, alpha-crystallin, is homologous to Hsp25 and its two subunits also have flexible C-terminal extensions. The flexible region in Hsp25 encompasses exactly that expected from sequence comparison with alpha-crystallin implying that both proteins have similar structures and that the C-terminal extensions could be of functional importance for both proteins.

Crystallization of the chaperone protein SecB

The secretory protein SecB found in Escherichia coli is a molecular chaperone that binds to precursor forms of a number of proteins targeted for export to the periplasmic space. SecB maintains these proteins in a translocation-competent conformation facilitating the translocation process. The material has been cloned and expressed in E. coli. Crystals have been grown from polyethylene glycol 8000 by vapor diffusion using the hanging drop technique. These crystals are monoclinic, belonging to space group C2 with unit cell dimensions a = 56.0 A, b = 111.1 A, c = 134.7 A, and beta = 104 degrees. The crystals diffract to 8 A resolution on a Rigaku imaging plate detector. Dynamic light scattering experiments suggest that SecB exhibits aggregation behavior with a number of different precipitating agents. These results may explain resistance of SecB to forming ordered crystals.