Immunological and biosynthetic studies of the human pyruvate dehydrogenase complex

coli at different temperatures, it was observed that both types of presequence as well as the nature of mature protein affect the level of protein expression. Therefore, it was noticed that both types of presequence had no significant effect on the level of protein expression when they were linked to the mature E3 whereas the negative effect of these presequences was apparent when they were linked to mature E2 or E3BP. Secondly, comparing the solubility of precursors with their mature forms, both extended and standard presequences markedly reduced the solubility of precursor constructs by inducing the production of inclusion bodies although the effect appeared to be more marked with the former. Thirdly, on decreasing the rate of the protein synthesis by growing E. coli cultures at lower temperatures, it was possible to minimise the formation of insoluble protein aggregates and achieve partial or indeed complete solubility of precursor forms in some cases. Fourthly, these precursors appeared to retain the ability to fold correctly or at least to initiate the correct folding pathway. Thus both soluble and insoluble fractions of E2 and E3BP precursors contained lipoylated domains as judged by their ability to cross-react with PD2, an indication that these N-terminally located domains had adopted their native conformations. These observations were consistent with the view that N-terminal mitochondrial targeting sequences markedly reduced the rate of protein folding rather than suppressing the folding process completely. In this scenario, precursors would exist as nascent folding intermediates for longer periods compared to their mature equivalents and so would be more prone to aggregation and degradation as observed in this study. Further experiments are planned to test this hypothesis.