The ParB protein, KorB, from the RK2 plasmid is required for DNA partitioning and transcriptional repression. It acts co-operatively with other proteins, including the repressor KorA. Like many multifunctional proteins, KorB contains regions of intrinsically disordered structure, existing in a large ensemble of interconverting conformations. Using NMR spectroscopy, circular dichroism and small-angle neutron scattering, we studied KorB selectively within its binary complexes with KorA and DNA, and within the ternary KorA/KorB/DNA complex. The bound KorB protein remains disordered with a mobile C-terminal domain and no changes in the secondary structure, but increases in the radius of gyration on complex formation. Comparison of wild-type KorB with an N-terminal deletion mutant allows a model of the ensemble average distances between the domains when bound to DNA. We propose that the positive co-operativity between KorB, KorA and DNA results from conformational restriction of KorB on binding each partner, while maintaining disorder.

A novel ELISA has been developed which detects oligomerization of β-amyloid (Aβ). Oligomerization, fibrillization and neurotoxicity of native Aβ associated with Alzheimer's disease (AD) type has been compared with E22Q Aβ (amyloid β-protein containing residues 1-40 with the native Glu at residue 22 changed to Gln) implicated in Dutch cerebral haemorrhage disease. Solutions of Aβ rapidly yield soluble oligomers in a concentration-dependent manner, which are detected by the ELISA, and by size-exclusion gel chromatography. Conformational changes from disordered to β-sheet occur more slowly than oligomerization, and fibrils are produced after prolonged incubation. The E22Q Aβ oligomerizes, changes conformation and fibrillizes more rapidly than the native form and produces shorter stubbier fibrils. Aged fibrillar preparations of E22Q Aβ are more potent than aged fibrils of native Aβ in inducing apoptotic changes and toxic responses in human neuroblastoma cell lines, whereas low-molecular-mass oligomers in briefly incubated solutions are much less potent. The differences in the rates of oligomerization of the two Aβ forms, their conformational behaviour over a range of pH values, and NMR data reported elsewhere, are consistent with a molecular model of oligomerization in which strands of Aβ monomers initially overcome charge repulsion to form dimers in parallel β-sheet arrangement, stabilized by intramolecular hydrophobic interactions, with amino acids of adjacent chains in register.