Ligand-induced conformational changes in proteins

The double role of methyl donor and allosteric effector of S-adenosyl-methionine for Dam methylase of E. coli

The turnover of DNA-adenine-methylase of E. coli strongly decreases when the temperature is lowered. This has allowed us to study the binding of Dam methylase on 14 bp DNA fragments at 0 degrees C by gel retardation in the presence of Ado-Met, but without methylation taking place. The enzyme can bind non-specific DNA with low affinity. Binding to the specific sequence occurs in the absence of S-adenosyl-methionine (Ado-Met), but is activated by the presence of the methyl donor. The two competitive inhibitors of Ado-Met, sinefungin and S-adenosyl-homocysteine, can neither activate this binding to DNA by themselves, nor inhibit this activation by Ado-Met. This suggests that Ado-Met could bind to Dam methylase in two different environments. In one of them, it could play the role of an allosteric effector which would reinforce the affinity of the enzyme for the GATC site. The analogues can not compete for such binding. In the other environment Ado-Met would be in the catalytic site and could be exchanged by its analogues. We have also visualized conformational changes in Dam methylase induced by the simultaneous binding of Ado-Met and the specific target sequence of the enzyme, by an anomaly of migration and partial resistance to proteolytic treatment of the ternary complex Ado-Met/Dam methylase/GATC.

The study of conformational states of proteins by nuclear magnetic resonance

By the use of examples, mainly of rather rigid proteins, we hope to have shown that conformational analysis of proteins is a problem that is not simply related to the conformational analysis of small molecules. The primary difficulties with proteins are (1) the multitude of possible conformers, (2) the complex dynamical behaviour and (3) the degree of co-operativity within the molecules. Any experimentally derived structural description of a protein is an attempt to represent some average of a complex time dependence. N.m.r. techniques have now reached the point where it is possible to use them to describe many detailed structural features of small globular proteins in solution and to detect and to describe conformational changes in such proteins. In addition, analysis is becoming possible of much less ordered regions of polypeptides, such as are found in less compact proteins, of for example myosin, histones and virus coat proteins, or in denatured states. The limits to the detailed conformational analysis of such proteins are likely to be ones of reality rather than method but the description of the properties shown in Table 1 is by its very nature an extremely important problem in conformational analysis of dynamic macromolecules.

A model for the non-specific binding of catabolite gene activator protein to DNA

The binding of E. coli catabolite gene activator protein (CAP) to non-specific sequences of DNA has been modelled as an electrostatic interaction between four basic side chains of the CAP dimer and the charged phosphates of DNA. Calculation of the electrostatic contribution to the binding free energy at various separations of the two molecules shows that complex formation is favored when CAP and DNA are separated by as much as 12 A. Thus, the long range electrostatic interactions may provide the initial energy for complex formation and also the correct relative orientation of CAP and DNA. The non-specific complex does not involve the penetration of amino acid side chains into the major grooves of DNA and permits 'sliding' of the protein along DNA, which would enhance the rate of association of CAP with the specific site as has been proposed previously for lac repressor. We propose that, as it 'slides', CAP is moving in and out of the major grooves in order to sample the DNA sequence. Recognition of the specific DNA site is achieved by a complementarity in structure and hydrogen bonding between amino acids and the edges of base pairs exposed in the major grooves of DNA.