Cross-bridge duty cycle in isometric contraction of skeletal myofibrils

During interaction of actin with myosin, cross-bridges impart mechanical impulses to thin filaments resulting in rotations of actin monomers. Impulses are delivered on the average every tc seconds. A cross-bridge spends a fraction of this time (ts) strongly attached to actin, during which it generates force. The "duty cycle" (DC), defined as the fraction of the total cross-bridge cycle that myosin spends attached to actin in a force generating state (ts/ tc), is small for cross-bridges acting against zero load, like freely shortening muscle, and increases as the load rises. Here we report, for the first time, an attempt to measure DC of a single cross-bridge in muscle. A single actin molecule in a half-sarcomere was labeled with fluorescent phalloidin. Its orientation was measured by monitoring intensity of the polarized TIRF images. Actin changed orientation when a cross-bridge bound to it. During isometric contraction, but not during rigor, actin orientation oscillated between two values, corresponding to the actin-bound and actin-free state of the cross-bridge. The average ts and tc were 3.4 and 6 s, respectively. These results suggest that, in isometrically working muscle, cross-bridges spend about half of the cycle time attached to actin. The fact that 1/ tc was much smaller than the ATPase rate suggests that the bulk of the energy of ATP hydrolysis is used for purposes other than performance of mechanical work.     

Structure of the O-polysaccharide and classification of Proteus mirabilis strain G1 in Proteus serogroup O3.

The O-chain polysaccharide of the lipopolysaccharide (LPS) of a previously nonclassified strain of Proteus mirabilis termed G1 was studied by sugar analysis and 1H and 13C NMR spectroscopy, including 2D COSY, TOCSY, rotating-frame NOE (ROESY), H-detected 1H,13C HMQC, and heteronuclear multiple-bond correlation (HMBC) experiments. The following structure of the polysaccharide was established: [carbohydrate structure: see text] where D-GalA6(L-Lys) stands for N(alpha)-(D-galacturonoyl)-L-lysine. The structure of the O-polysaccharide of P. mirabilis G1 is similar, but not identical, to that of P. mirabilis S1959 and OXK belonging to serogroup O3. Immunochemical studies with P. mirabilis G1 and S1959 anti-(O-polysaccharide) sera revealed close LPS-based serological relatedness of P. mirabilis G1 and S1959, and therefore it was suggested to classify P. mirabilis G1 in serogroup O3 as a subgroup. P. mirabilis G1 and S1959 anti-(O-polysaccharide) sera also cross-reacted with LPS of P. mirabilis strains from two other serogroups containing D-GalA6(L-Lys) in the O-polysaccharide or in the core region.     

Steady-state and time-resolved fluorescence studies of conformational changes induced by cyclic AMP and DNA binding to cyclic AMP receptor protein from Escherichia coli.

cAMP receptor protein (CRP), allosterically activated by cAMP, regulates the expression of several genes in Escherichia coli. As binding of cAMP leads to undefined conformational changes in CRP, we performed a steady-state and time-resolved fluorescence study to show how the binding of the ligand influences the structure and dynamics of the protein. We used CRP mutants containing a single tryptophan residue at position 85 or 13, and fluorescently labeled with 1,5-I-AEDANS attached to Cys178. Binding of cAMP in the CRP-(cAMP)2 complex leads to changes in the Trp13 microenvironment, whereas its binding in the CRP-(cAMP)4 complex alters the surroundings of Trp85. Time-resolved anisotropy measurements indicated that cAMP binding in the CRP-(cAMP)2 complex led to a substantial increase in the rotational mobility of the Trp13 residue. Measurement of fluorescence energy transfer (FRET) between labeled Cys178 and Trp85 showed that the binding of cAMP in the CRP-(cAMP)2 complex caused a substantial increase in FRET efficiency. This indicates a decrease in the distance between the two domains of the protein from 26.6 A in apo-CRP to 18.7 A in the CRP-(cAMP)2 complex. The binding of cAMP in the CRP-(cAMP)4 complex resulted in only a very small increase in FRET efficiency. The average distance between the two domains in CRP-DNA complexes, possessing lac, gal or ICAP sequences, shows an increase, as evidenced by the increase in the average distance between Cys178 and Trp85 to approximately 20 A. The spectral changes observed provide new structural information about the cAMP-induced allosteric activation of the protein.     

Modeling the formation of root apices

The formation of new root apices from small groups of cells with different cellular patterns has been simulated using an existing model based on growth tensors. To generate an apex, a steady growth field was used. The pattern of cells evolved to approach the steady state. Two extreme types of progressions have been obtained : one leading to an apex with a single or a few apical cells, and the other to an apex with a quiescent centre. The change of structure while applying a steady growth tensor indicates that development may involve a succession of discrete growth tensors.     

Analysis of immune response: comparison of immunoblots after isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis using cytoplasmic protein extract from Brucella

Analysis of the immune response towards the facultative intracellular bacterium, Brucella melitensis, was studied by immunoblotting after either isoelectric focusing (IEF) or sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). A cytoplasmic extract (CPE) of Brucella melitensis was used as antigen to analyse the response in 17 sera from naturally infected goats. CPE analysed by IEF exhibited 25 proteins within the pH range of 4.35 to 6. Immunoblotting revealed most of the stained bands around pH 4.5-5.4. CPE analysed by SDS-PAGE showed more than 20 silver stained proteins in the molecular range of 16-18 kDa to 70 kDa but immunoblotting revealed only 1 to 6 bands according to the sera tested. Because proteins are preserved in their native state with IEF, in contrast to SDS-PAGE treatment, this technique may be best suited for analysis of the overall response to natural infection.     

Reversible covalent binding of peptide nitriles to papain

The dissociation constants for reversible covalent binding of twelve peptide nitrile inhibitors to the active site of papain have been measured by means of fluorescence titration. The binding constants generally parallel the kinetic specificity constants (kcat/Km) for related papain substrates, supporting earlier suggestions that peptide nitriles behave as transition state analog inhibitors of papain. In ten cases the temperature dependence of binding was analyzed to determine the enthalpic and entropic contributions to the binding energy. A compensation plot of delta H vs. T delta S resulted in two parallel lines, one for 'specific' nitriles (i.e., N-Ac-L-aa-NHCH2CN; aa = Phe, Leu, Met) and the other for 'non-specific' nitriles (e.g., N-Ac-D-Phe-NHCH2CN, PhCH2CH2CONHCH2CN hippurylnitrile, etc.). For both specific and nonspecific nitriles representing an 1800-fold range of Kd values (0.27 microM-490 microM), the solvent deuterium isotope effect on binding (Kd(H2O)/Kd(D2O) = DKd) was very close to 2.0. This isotope effect could be accounted for entirely by the simple protonic change which occurs upon the reversible addition of the active site sulfhydryl of papain to the nitrile group of the peptide derivative to form a covalent thioimidate linkage. In contrast, six closely related non-nitrile ligands containing identical peptide side chains but having C-terminal groups incapable of binding covalently to papain had unmeasureably high dissociation constants. Collectively, these results indicate that strong binding of peptide nitrile substrate analogs to papain requires a combination of (1) hydrophobic interaction (especially at the P2 position), (2) specific intermolecular hydrogen bonding and (3) covalent interaction of the nitrile with the active site sulfhydryl group.