Observations that ␤-sheet proteins form amyloid fibrils under at least partially denaturing conditions has raised questions as to whether these fibrils assemble by docking of preformed ␤-structure or by association of unfolded polypeptide segments. By using ␣-helical protein apomyoglobin, we show that the ease of fibril assembly correlates with the extent of denaturation. By contrast, monomeric ␤-sheet intermediates could not be observed under the conditions of fibril formation. These data suggest that amyloid fibril formation from apomyoglobin depends on disordered polypeptide segments and conditions that are selectively unfavorable to folding. However, it is inevitable that such conditions often stabilize protein folding intermediates.

The refolding of apo bovine alpha-lactalbumin has been monitored in real time by NMR spectroscopy following rapid in situ dilution of a chemically denatured state. By examining individual resonances in the time-resolved NMR spectra, the native state has been shown to emerge in a cooperative manner from an intermediate formed in the dead-time of the experiments. The kinetics of folding to the native state are closely similar to those observed by stopped-flow fluorescence and near-UV circular dichroism. The NMR spectrum of the transient intermediate resembles closely that of the well characterized stable molten globule state formed at low pH. The results suggest that NMR can play a key role in describing at an atomic level the structural transitions occurring during protein folding.

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The study of the membrane insertion of the translocation domain of diphtheria toxin deepens our insight into the interactions between proteins and membranes. During cell intoxication, this domain undergoes a change from a soluble and folded state at alkaline pH to a functional membrane-inserted state at acid pH. We found that hydrophobic and electrostatic interactions occur in a sequential manner between the domain and the membrane during the insertion. The first step involves hydrophobic interactions by the C-terminal region. This is because of the pH-induced formation of a molten globule specialized for binding to the membrane. Accumulation of this molten globule follows a precise molecular mechanism adapted to the toxin function. The second step, as the pH decreases, leads to the functional inserted state. It arises from the changes in the balance of electrostatic attractions and repulsions between the N-terminal part and the membrane. Our study shows how the structural changes and the interaction with membranes of the translocation domain are finely tuned by pH changes to take advantage of the cellular uptake system.Folding and insertion of membrane proteins (1, 2), binding of hormones to membrane receptors (3), action of antibiotic peptides (4, 5), protein translocation, and internalization of toxins (6) are examples of phenomena that require the interactions of polypeptide chains with membranes. Because of the anisotropic nature of membranes, the initial steps of the association and the final structure and localization of polypeptide chains within membranes depend on a combination of hydrophobic and electrostatic interactions (7). Hydrophobic interactions are dominant for the insertion of transmembrane polypeptides. Electrostatic interactions are important for the binding of antibiotic peptides (5,8), the association of proteins with the surface of the membrane (9 -11), and as determinants of the topology of integral membrane proteins after biosynthesis (12). In most cases, electrostatic interactions are the result of the attraction between anionic phospholipid head groups and basic amino acid side chains (13,14). However, there are examples where electrostatic repulsions are involved in the membrane association of peptides, particularly when their structure and localization within the membrane is regulated by the pH (15-19). The interplay of hydrophobicity and electrostatics and their distribution within the polypeptide sequence have only been studied in detail for small peptides (3-5, 7, 13-15, 17-20). In the case of proteins, the role of these effects on the association with and the insertion into membranes are still poorly understood (2,(21)(22)(23)(24).The study of the membrane insertion process of the translocation (T) 1 domain of diphtheria toxin (25) can provide precious insight into the interactions between proteins and membranes and the refolding mechanisms of membrane proteins. During intoxication of cells (25), the toxin reaches the early endosomes through the clathrin-coated pathway (26). Beca...

An approach is described to monitor directly at the level of individual residues the formation of structure during protein folding. A two-dimensional heteronuclear nuclear magnetic resonance (NMR) spectrum was recorded after the rapid initiation of the refolding of a protein labeled with nitrogen-15. The intensities and line shapes of the cross peaks in the spectrum reflected the kinetic time course of the folding events that occurred during the spectral accumulation. The method was used to demonstrate the cooperative nature of the acquisition of the native main chain fold of apo bovine alpha-lactalbumin. The general approach, however, should be applicable to the investigation of a wide range of chemical reactions.