THE EXPRESSION OF BOVINE MICROSOMAL CYTOCHROME B5 IN ESCHERICHIA-COLI AND A STUDY OF THE SOLUTION STRUCTURE AND STABILITY OF VARIANT PROTEINS

The DNA sequence of bovine microsomal cytochrome b5 has been amplified from a liver cDNA library using a polymerase chain reaction. The amplified cDNA when cloned into plasmids that support the high-level production of cytochrome b5 in E. coli leads to protein overexpression and results in cell colonies bearing a strong red colouration. Using cassette mutagenesis, truncated versions of the cytochrome b5 cDNA have been made that encode the first 90 amino acid residues (Ala1 - Lys90), the first 104 amino acids (Ala1 - Ser104) and the complete protein (Ala1 - Asn133). The location of the overexpressed cytochrome b5 within prokaryotic cells is dependent on the overall length of the protein. Expression of the Ala-Lys90 and Ala1-Ser104 variants leads to a location in the cytoplasmic phase of the bacteria whereas the whole protein, Ala1 - Asn133, is found within the bacterial membrane fraction. The last 30 residues of cytochrome b5 therefore contain atl of the necessary information to insert the protein into E. coli membranes. The solubility of the Ala1 - Ser104 variant permits the solution structure and stability of this protein to be measured using 1- and 2-D H-1-NMR methods and electronic spectroscopy. 1-D NMR studies show that the chemical shifts of the haem and haem ligand resonances of the Ala1 - Ser104 variant exhibit only very slight perturbations to their magnetic microenvironments when compared with the tryptic fragment of ferricytochrome b5. These results indicate an arrangement of residues in the haem pocket that is very similar in both the Ala1 - Ser104 variant and the tryptic fragment and by 2-D NMR it is shown that this similarity extends to the conformations of the polypeptide backbone and side chains. Electronic spectroscopy of this variant shows absorbance maxima for the Soret peaks at 423 nm (reduced) and 413 nm (oxidized). From absorbance spectra the relative thermal stabilities of the Ala1-Ser104 variant and the tryptic fragment were measured. In the oxidized state the Ala1 - Ser104 variant denatures in a single cooperative transition with a midpoint temperature (T(m)) of 73-degrees-C that is significantly higher than that of 'tryptic' ferricytochrome b5. The reduced form of the protein shows increased transition temperatures (T(m) approximately 78-degrees-C) reflected in the values of DELTAH(m), DELTAS(m) and DELTA(DELTAG) of 420 kJ/mol, 1096 J/mol/K and 12.38 kj/mol respectively, estimated for this variant. The increased stability of the Ala1 - Ser104 variant and other recombinant forms of cytochrome b5 is correlated with the presence of additional residues at the N- and C-termini. The subtle differences in reactivity, stability and targeting between variant forms of cytochrome b5 and the tryptic fragment are discussed in terms of the overall structure of the protein.