Cytochrome c from Schizosaccharomyces pombe. 1. Purification, spectral properties, and amino-acid composition.

Cytochrome c from the fission yeast Schizosaccharomyces pombe has been purified. Its chromatographic and spectral properties are reported and compared to those of iso-1-cytochrome c from baker's yeast; the amino-acid composition is described. Schiz. pombe cytochrome c has a much lower affinity for Amberlite IRP64 than Sacch. cerevisiae iso-1-cytochrome c. Its alpha absorption band splits into three maxima (calpha1, calpha2, and calpha3) at -190 degrees C; this is unusual in yeasts, as shown by the low-temperature whole-cell absorption spectra which were examined in various yeast genera, species, and strains. A minor component can be separated by Amberlite chromatography. It exhibits the same low-temperature splitting of the alpha absorption band as the main fraction and it has a similar amino-acid composition with a notable exception: it is an unmethylated form of the cytochrome.

Cytochrome c from Schizosaccharomyces pombe. 2. Amino-acid sequence.

The amino acid sequence of Schizosaccharomyces pombe cytochrome c has been established by automatic degradation of the protein and by manual degradation of fragments obtained by cyanogen bromide cleavage and chymotryptic digestion. The chymotryptic peptides were aligned by homology with other known cytochrome c sequences. The protein is 108 residues long, with a four-residue amino-terminal tail. It has only one methionine residue and differs from other fungal cytochromes c in lacking the one-residue deletion at the C-terminal end. After a cyanogen bromide step, an unexpected cleavage of the peptide chain before a cysteine residue was observed. This is ascribed to formation of a dehydroalanyl residue during an incomplete S-carboxymethylation of the apoprotein, and subsequent cleavage under acidic conditions. Experimental evidence is presented in favour of the proposed mechanisms.