Targeting of a mitochondrial intermembrane space protein: flavocytochrome b₂

Abstract

During their biogenesis, mitochondria import approximately 90% of their polypeptides from the cytoplasm. The majority of these proteins are synthesised as larger molecular weight precursors containing a targeting signal or presequence at their N-terminus. This presequence, usually 20-80 amino acids in length, directs the precursor to the mitochondrion and is proteolytically removed during the import process. While recent studies have determined many of the features associated with the import process, current work is focused on the molecular characterisation of components of the import apparatus and of precursor proteins themselves.

The work presented in this thesis describes an attempt to generate large quantities of the purified precursor of flavocytochrome b₂ a, mitochondrial intermembrane space protein from the yeast Saccharomyces cerevisiae, by its high-level expression in both yeast and Escherichia coli. A comparison between the expression of this precursor and the corresponding mature protein on the effect of growth of E.coli is made and the initial steps involved in purifying the precursor from the bacterium are described.

In addition, the first of two proteolytic cleavage sites is identified by constructing a modified form of the precursor protein by sitedirected mutagenesis of the CYB2 gene. This modified form accumulates in yeast mitochondria as an intermediate molecular weight species and the first cleavage site was identified by determining the N-terminal amino acid sequence of the intermediate form.

Finally, the targeting abilities of the remaining N-terminal presequence after the first cleavage has occurred are examined and the results discussed in relation to theories on the evolution of protein targeting pathways within mitochondria.