Damage increases the flexibility of duplex DNA. (1/620)

It is proposed that much of the recognition of specific types of damaged DNAs is based on accessible structural features, while much of the recognition of damaged DNAs, as a class, is based on flexibility. The more flexible a DNA the faster its diffusion rate. The diffusion rates of each member of a series of damaged duplex DNAs has been found to be significantly faster than that of the corresponding undamaged duplex DNA. The damaged sites studied include apurinic and apyrimidinic a basic sites, thymine glycol and urea. The presence of mismatched sites also increases the diffusion. Thus, damaged DNAs appear to have sufficient flexibility for recognition and the flexibility may allow damaged sites to act as a universal joint or hinge that allows distant sites on the DNA to come together.  (+info)

Functional organization of clathrin in coats: combining electron cryomicroscopy and X-ray crystallography. (2/620)

The sorting of specific proteins into clathrin-coated pits and the mechanics of membrane invagination are determined by assembly of the clathrin lattice. Recent structures of a six-fold barrel clathrin coat at 21 A resolution by electron cryomicroscopy and of the clathrin terminal domain and linker at 2.6 A by X-ray crystallography together show how domains of clathrin interact and orient within the coat and reveal the strongly puckered shape and conformational variability of individual triskelions. The beta propeller of the terminal domain faces the membrane so that recognition segments from adaptor proteins can extend along its lateral grooves. Clathrin legs adapt to different coat environments in the barrel by flexing along a segment at the knee that is free of contacts with other molecules.  (+info)

The structural flexibility of the preferredoxin transit peptide. (3/620)

In order to obtain insight into the structural flexibility of chloroplast targeting sequences, the Silene pratensis preferredoxin transit peptide was studied by circular dichroism and nuclear magnetic resonance spectroscopy. In water, the peptide is unstructured, with a minor propensity towards helix formation from Val-9 to Ser-12 and from Gly-30 to Ser-40. In 50% (v/v) trifluoroethanol, structurally independent N- and C-terminal helices are stabilized. The N-terminal helix appears to be amphipathic, with hydrophobic and hydroxylated amino acids on opposite sides. The C-terminal helix comprises amino acids Met-29-Gly-50 and is destabilized at Gly-39. No ordered tertiary structure was observed. The results are discussed in terms of protein import into chloroplasts, in which the possible interactions between the transit peptide and lipids are emphasized.  (+info)

The length of the flexible SNAREpin juxtamembrane region is a critical determinant of SNARE-dependent fusion. (4/620)

The topology of a SNARE complex bridging two docked vesicles could act as a mechanical couple to do work on the lipid bilayer resulting in fusion. To test this, we prepared a series of modified SNARE proteins and determined their effects on SNARE-dependent membrane fusion. When two helix-breaking proline residues are introduced into the juxtamembrane region of VAMP, there is little or no effect on fusion, and the same change in syntaxin 1A only reduced the extent and rate of fusion by half. The insertion of a flexible linker between the transmembrane domain and the conserved coiled-coil domain only moderately affected fusion; however, fusion efficiency systematically decreased with increasing length of the linker. Together, these results rule out a requirement for helical continuity and suggest that distance is a critical factor for membrane fusion.  (+info)

Actin activates a cryptic dimerization potential of the vinculin tail domain. (5/620)

The tail domain of vinculin (V(t)) is an actin binding module containing two regions that interact with F-actin. Although intact V(t) purified from a bacterial expression system is a globular monomer, each actin binding region dimerizes when expressed individually, suggesting the presence of cryptic self-association sites whose exposure is regulated. We show that actin modulates V(t) self-association by inducing or stabilizing a conformational change in V(t) that allows dimerization. Chemical cross-linking studies implicate one of the actin binding regions in mediating dimerization in the presence of actin. Actin-induced V(t) dimers may play a role in the filament cross-linking activity of this protein. The V(t) dimers induced by actin are biochemically distinct from the V(t) dimers and higher oligomers induced by acidic phospholipids such as phosphatidylinositol 4,5-bisphosphate, suggesting structural differences in V(t) bound to these two ligands that may provide a mechanistic basis for inhibition of F-actin binding by phosphatidylinositol 4,5-bisphosphate. The ability of actin to regulate the dimerization state of an actin binding protein suggests that, rather than serving a passive structural role, actin filaments may directly participate in signal transduction and other cellular events that are known to depend on cytoskeletal integrity.  (+info)

In vitro studies on the maintenance of transcription-induced stress by histones and polyamines. (6/620)

Several factors were evaluated to determine their role in facilitating the presence of transcription-induced stresses in a circular DNA. Transcription was done with T7 RNA polymerase in the presence of E. coli topoisomerase I and closed circular DNA. Positive stress was observed in hypotonic conditions or when one of the polyamines, spermidine or spermine, were present. Polycations such as polylysine, polyarginine, histone H1, histones H2A-H2B, and protamine were observed to induce minimal positive stress. It is known that polyamines influence DNA structure by causing both self-association and sequence-specific structural alterations (polyamine-induced localized bending). Experimental evidence indicates that the likely cause of the positive stress is the induced bending. In order to evaluate protein-mediated bending, transcription was done on nucleosomes. A minimum of three nucleosomes on a DNA of 6055 bp was sufficient to generate very high levels of positive stress. Histones H3-H4 in the absence of H2A-H2B were responsible for this effect. Since these histones by themselves are able to maintain negative coils on DNA, it is concluded that protein-mediated bending is yet another mechanism for placing rotational restriction on DNA. The bending of DNA by either polyamines or histones is an effective mechanism for promoting transcription-induced stresses at physiological ionic strength.  (+info)

Balance control, flexibility, and cardiorespiratory fitness among older Tai Chi practitioners. (7/620)

BACKGROUND: Tai Chi Chuan (TTC) exercise has beneficial effects on the components of physical condition and can produce a substantial reduction in the risk of multiple falls. Previous studies have shown that short term TCC exercise did not improve the scores in the single leg stance test with eyes closed and the sit and reach test. There has apparently been no research into the effects of TCC on total body rotation flexibility and heart rate responses at rest and after a three minute step test. METHODS: In this cross sectional study, 28 male TCC practitioners with an average age of 67.5 years old and 13.2 years of TCC exercise experience were recruited to form the TCC group. Another 30 sedentary men aged 66.2 were selected to serve as the control group. Measurements included resting heart rate, left and right single leg stance with eyes closed, modified sit and reach test, total body rotation test (left and right), and a three minute step test. RESULTS: Compared with the sedentary group, the TCC group had significantly better scores in resting heart rate, three minute step test heart rate, modified sit and reach, total body rotation test on both right and left side (p < 0.01), and both right and left leg standing with eyes closed (p < 0.05). According to the American Fitness Standards, the TCC group attained the 90th percentile rank for sit and reach and total body rotation test, right and left. CONCLUSION: Long term regular TCC exercise has favourable effects on the promotion of balance control, flexibility, and cardiovascular fitness in older adults.  (+info)

The relationship of lumbar flexion to disability in patients with low back pain. (8/620)

BACKGROUND AND PURPOSE: Physical therapists routinely assess spinal active range of motion (AROM) in patients with low back pain (LBP). The purpose of this study was to use 2 approaches to examine the relationship between impairment of lumbar spine flexion AROM and disability. One approach relied on the use of normative data to determine when an impairment in flexion AROM was present. The other approach required therapists to make judgments of whether the flexion AROM impairment was relevant to the patient's disability. SUBJECTS: Fifteen physical therapists and 81 patients with LBP completed in the study. METHODS: Patients completed the Roland-Morris Back Pain Questionnaire (RMQ), and the therapists assessed lumbar spine flexion AROM using a dual-inclinometer technique at the initial visit and again at discharge. RESULTS: Correlations between the lumbar flexion AROM measure and disability were low and did not vary appreciably for the 2 approaches tested. CONCLUSION AND DISCUSSION: Measures of lumbar flexion AROM should not be used as surrogate measures of disability. Lumbar spine flexion AROM and disability are weakly correlated, suggesting that flexion AROM measures should not be used as treatment goals.  (+info)

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