In-vitro Studies on the Folding Characteristics of the Escherichia Coli Precursor Protein PrePhoE. Evidence That SecB Prevents the Precursor from Aggregating by Forming a Functional Complex

We characterised the behaviour of the purified precursor protein prePhoE upon dilution from 8 M urea by CD, fluorescence spectroscopy and gel-filtration techniques. It is demonstrated that prePhoE rapidly adopts beta structure, folds and aggregates upon dilution to urea concentrations below 3 M. These processes are paralleled by a loss of translocation competence. Furthermore the interaction of prePhoE with SecB was investigated. SecB is shown to have a very high content of beta structure, therefore we propose that precursor recognition by SecB is mediated through beta-beta interaction. It is shown that SecB has little effect on the adoption of secondary structure and tertiary folding upon dilution of the precursor from urea. However, SecB prevents the precursor from aggregating by forming a functional and stable complex.