The Burst-phase Intermediate in the Refolding of β-Lactoglobulin Studied by Stopped-flow Circular Dichroism and Absorption Spectroscopy

The kinetics of the guanidine hydrochloride-induced unfolding and refolding of bovine β-lactoglobulin, a predominantly β-sheet protein in the native state, have been studied by stopped-flow circular dichroism and absorption measurements at pH 3.2 and 4.5°C. The refolding reaction was a complex process composed of different kinetic phases, while the unfolding was a single-phase reaction. Most notably, a burst-phase intermediate of refolding, which was formed during the dead time of stopped-flow measurements (∼18 ms), showed more intense ellipticity signals in the peptide region below 240 nm than the native state, yielding overshoot behavior in the refolding curves. We have investigated the spectral properties and structural stability of the burst-phase intermediate and also the structural properties in the unfolded state in 4.0 M guanidine hydrochloride of the protein and its disulfide-cleaved derivative. The main conclusions are: (1) the more intense ellipticity of the intermediate in the peptide region arises from formation of non-native α-helical structure in the intermediate, apparently suggesting that the folding of β-lactoglobulin is not represented by a simple sequential mechanism. (2) The burst-phase intermediate has, however, a number of properties in common with the folding intermediates or with the molten globule states of other globular proteins whose folding reactions are known to be represented by the sequential model. These properties include: the presence of the secondary structure without the specific tertiary structure; formation of a hydrophobic core; broad unfolding transition of the intermediate; and rapidity of formation of the intermediate. The burst-phase intermediate of β-lactoglobulin is thus classified as the same species as the molten globule state. (3) The circular dichroism spectra of β-lactoglobulin and its disulfide-cleaved derivative in 4.0 M guanidine hydrochloride suggests the presence of the residual β-structure in the unfolded state and the stabilization of the β-structure by disulfide bonds. Thus, if this residual β-structure is part of the native β-structure and forms a folding initiation site, the folding reaction of β-lactoglobulin may not necessarily be inconsistent with the sequential model. The non-native α-helices in the burst-phase intermediate may be formed in an immature part of the protein molecule because of the local α-helical propensity in this part

Rapid formation of secondary structure framework in protein folding studied by stopped-flow circular dichroism

Kinetic refolding reactions of ferricytochrome c and β-lactoglobulin have been studied by stopped-flow circular dichroism by monitoring rapid ellipticity changes of peptide backbone and side-chain chromophores. In both proteins, a transient intermediate accumulates within the dead time of stopped-flow mixing (18 ms), and the intermediate has an appreciable amount of secondary structure but possesses an unfolded tertiary structure. It is suggested that the rapid formation of a secondary structure framework in protein folding is a common property observed in a variety of globular proteins

FBP17 Mediates a Common Molecular Step in the Formation of Podosomes and Phagocytic Cups in Macrophages*S⃞

Macrophages act to protect the body against inflammation and infection by engaging in chemotaxis and phagocytosis. In chemotaxis, macrophages use an actin-based membrane structure, the podosome, to migrate to inflamed tissues. In phagocytosis, macrophages form another type of actin-based membrane structure, the phagocytic cup, to ingest foreign materials such as bacteria. The formation of these membrane structures is severely affected in macrophages from patients with Wiskott-Aldrich syndrome (WAS), an X chromosome-linked immunodeficiency disorder. WAS patients lack WAS protein (WASP), suggesting that WASP is required for the formation of podosomes and phagocytic cups. Here we have demonstrated that formin-binding protein 17 (FBP17) recruits WASP, WASP-interacting protein (WIP), and dynamin-2 to the plasma membrane and that this recruitment is necessary for the formation of podosomes and phagocytic cups. The N-terminal EFC (extended FER-CIP4 homology)/F-BAR (FER-CIP4 homology and Bin-amphiphysin-Rvs) domain of FBP17 was previously shown to have membrane binding and deformation activities. Our results suggest that FBP17 facilitates membrane deformation and actin polymerization to occur simultaneously at the same membrane sites, which mediates a common molecular step in the formation of podosomes and phagocytic cups. These results provide a potential mechanism underlying the recurrent infections in WAS patients