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The last 10 years have seen the development of a large body of knowledge dealing with problems of the assembly of biological macromolecules into specific ordered structures. The ideas which have emerged from these studies may be regarded as an extension of the well established principle that the sequence of amino acid residues in a polypeptide chain is the main determinant of the complex folding of the chain into the native protein conformation. The concept of “self-folding” of the polypeptide chain was a natural starting point for dealing with the question of how macromolecules assemble themselves into large ordered structures for it introduced the idea that seemingly minor alterations in the microstructure of molecules can induce profound changes in their macrostructure.

1. 1.|Linear self-association of bovine liver glutamate dehydrogenase ( l -glutamate NAD(P) + oxidoreductase (deaminating), EC 1.4.1.3) is considerably enhanced by saturation with toluene of buffered enzyme solutions (0.2 M sodium phosphate buffer (pH 7), 10 −4 M EDTA). The association was followed by light scattering studies and can be described by a single association constant, K , equal to 52, 28.8 and 21.7 ml/mg at 10, 20 and 25°, respectively (the value of K at 25° in the absence of toluene is 2 ml/mg); benzene, which is more soluble in aqueous solutions than toluene, enhances glutamate dehydrogenase association even more. 2. 2.|Solubility studies of toluene in buffered glutamate dehydrogenase solutions, saturated with respect to toluene, may be interpreted to indicate that 8–13 moles of toluene are preferentially bound per 53 500 g of enzyme; below saturation conditions considerably less toluene is preferentially bound to the enzyme. The biological activity of glutamate dehydrogenase is not affected by toluene binding. The binding increases with decreasing temperature and increasing degree of association. It was found that toluene solubility is not increased by glutamate dehydrogenase in buffered solution containing 5·10 −3 M GTP, 5·10 −3 M NADH; in the presence of these effectors both biological activity and association of the enzyme are much reduced.

It is suggested that under physiological conditions (> 1 mM Mg 2+ ) MgATP binds to myosin to form a chelate involving the two reactive sulfhydryl sites (SH 1 and SH 2 ). The stability of the chelate structure results in marked inhibition of the myosin ATPase in the presence of millimolar magnesium ion. The inhibitory effect of magnesium ion can be eliminated chemically by blocking either the SH 1 or SH 2 site since this precludes formation of the chelate. In muscle, actin apparently behaves in a similar fashion in that its interaction with myosin causes a disruption of the chelate structure.

The angular dependence of scattering in solutions of bovine liver glutamate dehydrogenase was studied in 0.2M Na-phosphate buffer, pH 7, 10−4M EDTA, and in buffered solutions saturated with either toluene or benzene. Whereas the shape of the scattering curves is in qualitative accord with the previously reported polymerization to rodlike linear structures, quantitative agreement between the calculated and experimental curves is poor when the polydispersity consistent with the association mechanism (derived from molecular weight determinations by light scattering and equilibrium sedimentation) is properly taken into account. The measurement of rotary frictional coefficients of macroscopic models of linearly associated glutamate dehydrogenase oligomers allowed the calculation of intrinsic viscosities of enzyme solutions but did not permit discrimination, by hydrodynamic means, between systems with different distributions of length.

The refractive indices and densities of benzene, methanol, and carbon tetrachloride have been determined in the temperature range between 8 and 50°C. The data were fitted to an empirical extension of the Lorentz-Lorenz equation previously proposed, which takes into account the change of the polarizability α with temperature and density. The density dependence of α at constant temperature (piezo-optic effect) for the three liquids is quite similar to that previously observed for water and heavy water, and may be qualitatively understood in terms of changes in molecular electronic structure upon compression. In contrast to water and heavy water, which exhibit a significant thermo-optic effect, α is essentially independent of temperature at constant density for all three liquids studied here. A model based on the temperature dependence of librational motion in the liquid appears to account for what now seems to be the anomalously large thermo-optic effect in water and heavy water.

Drebrin A is neuronal F-actin-binding protein enriched in dendritic spines, which are playing a major role in synaptic plasticity. Molecular cross-talk between drebrin and key neuronal actin-binding proteins is believed to be a part of the cytoskeleton-modulating pathway that is implicated also in synaptic dysfunction in Alzheimer's disease (AD), Down syndrome, epilepsy and normal aging. However, drebrin effects on actin dynamics and its interplay with main neuronal ABPs (such as severing proteins, cappers, elongation factors) are yet to be clarified. Here, we show that drebrin A is a leaky capper of actin filaments. Solution assays monitoring pyrene fluorescence of actin indicate that drebrin affects barbed- but not pointed end polymerization. Real time TIRF assays revealed >2-fold inhibition of F-actin elongation rate by drebrin A. Such inhibition was observed also in the presence of profilin. Moreover, consistent with its capping activity, drebrin inhibits F-actin annealing. These results called for examination of drebrin effects on barbed end binding factors, such as formins. Pyrene fluorescence assays show inhibition of formin-induced actin polymerization by drebrin A in the presence of profilin. The mechanism of such inhibition and its biological implications will be discussed.

Cofilin/ADF proteins play key roles in the dynamics of actin. We used cryo-electron microscopy of uniformly decorated actin-cofilin filaments to show that the cofilin induced change in the filament twist is due to a unique conformation of the actin molecule unrelated to any previously observed state. The changes between the actin protomer in naked F-actin and in the actin-cofilin filament are greater than the conformational changes between G- and F-actin. Cofilin/ADF proteins efficiently depolymerize F-actin only when bound at low stoichiometry to actin filaments. We also used cryo electron microscopy to reveal the structure of F-actin decorated with sub-stoichiometric amounts of cofilin. Our results suggest that the structural state of actin protomers found within the uniformly decorated actin-cofilin filaments can propagate towards the naked regions, and this cooperative propagation is uncoupled from the change in the helical twist of F-actin upon interaction with cofilin. This illustrates the structural plasticity of actin and provides a structural mechanism for actin depolymerization by the ADF/cofilin proteins.