Loose folds

Place 100 g of thin aluminium foil (c. 0.05 mm thickness), cut into strips about 15 cm long and 2.5 cm wide and loosely folded, in a 3-litre flask and cover with a 10 per cent solution of sodium hydroxide warm the flask on a water bath until a vigorous evolution of hydrogen has taken place for several minutes (CAUTION). Wash the foil thoroughly with water and with rectified spirit this... 

Set B was prepared in substantially the same way, but with minor alterations. Larger samples were used, 15 cm by 91 cm. The samples were loosely folded and allowed to soak in the stabilizer solution for 30 minutes, then rinsed with distilled/deionized water to remove any stabilizer left on the surface. The fabrics were then hung over a glass rod and allowed to air dry. Again, the add-ons were much closer to those desired (Table IV). 

It is to be noted that the reflection assigned to the new phase in butyl branched alkanes is relatively weak compared to the reflections observed for the new phase in ethylene-1-octene copolymer (5.2 mol %). As explained in this chapter, we attribute the new phase to the crystallization of transient layer (butyl branches and fold surface). Considering the anticipated tight folds for butyl branched alkanes, the amount of crystallizable entities in the branched alkanes would be much less than in the ethylene-1-octene copolymers where the loose folds are expected. We would like to mention that, considering the d-value and intensity of the pseudo-hexagonal phase in branched alkanes, this reflection may be referred to as open-orthorhombic phase. 

The molecular chaperone SecB binds a subset of Escherichia coli envelope protein precursors and promotes their efficient export from the cytoplasm by (1) maintaining them in a loosely folded or export-competent conformation, (2) blocking nonproductive interactions with the cytoplasmic membrane, (3) preventing aggregation, and (4) targeting them to SecA, the peripheral membrane component of the preprotein translocase. 

A growing body of evidence indicates that in addition to a targeting element, membrane translocation also requires that the secretory protein assumes a loosely folded or translocation-competent state. The focus of this review is the role that SecB and other molecular chaperones play in sponsoring efficient protein secretion in E. coli. 

If export competence is associated with a loosely folded precursor, then parameters that accelerate folding or stabilize folded states should impede export. This relationship has also been explored in E. coli using a DHFR fusion protein. At low levels of synthesis, a hybrid protein consisting of the signal peptide and the first 153 amino acid residues of OmpA joined to DHFR is efficiently secreted. However, addition of trimethoprim imparts a kinetic defect in the secretion rate of the hybrid protein. The effect of trimethoprim is dependent on the presence of a full length, presumably active, DHFR moiety, indicating that secretion in vivo is inhibited by stabilization of the native DHFR structure (FreudI et ai, 1988). 

Throughout this review, terms such as unfolded, loosely folded, or export-competent state are bandied about (no doubt distressing some disciples of protein structure). These terms are not meant to imply a single definable conformation, or the absence of a specific secondary or tertiary structure. In fact, several lines of evidence indicate that a translocation-competent precursor can contain considerable structure. For example, efficient signal peptide function requires that the hydrophobic core assume an a-helical conformation (Jones et ai, 1990), and evidence has been presented that enzymatically active DHFR (Freudl et al., 1988) and a fully folded biotin acceptor domain (Reed and Cronan, 1991)—or conformers of these proteins that are in equilibrium with the native states—can be translocated across the cytoplasmic membrane of E. coli in vivo. 

Proproteins interact with molecular chaperones in loosely-folded, translocation-competent conformations, particularly for a polypeptide chain targeted for the post-translalional process. 

The regular folded array with adjacent reentry of the ehains, but with some loose folding and emergent chain ends or cilia that contribute to the disordered surface... 

The precursor proteins are maintained in a translocation-competent state by the folding-antagonizing activity of their signal peptides and cytoplasmic chaperones, which potentially include the signal recognition particle (SRP) and the FtsY protein [47-49]. This enables translocation across the membrane through the Sec apparatus, which can only handle unfolded or loosely folded proteins. 

The sketch includes amcrphcus areas (defects), chain ends, rotated crystals, loose folds, sharp folds, areas of para-crystalline structure, extended-chain sections, voids, sheared regions, chain-kinks, single fibrils, migrating folds, and single-crystal regicns. 

Fraction of sites with crystallographic folds Fraction of sites with loose folds Length factor for free ends s Length factor for loose folds s Relative probability of connector types ... 

Boyd speculates that the shortest relaxation times may be associated with motions of very loose folds and relatively non-extended tie chains conversely tight folds are unable to relax. The relative prominence of the relaxation in LDPE compared with HDPE is enhanced by the lower value of the relaxed fi process modulus in LDPE, which will increase the relative intensity of the fi and decrease that of the a. On a molecular basis the branching of LDPE gives a more loosely organized amorphous component, capable of relaxing to a lower limiting rubbery modulus. 

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