Stabilizing amino acid replacements at position 52 in yeast iso-1-cytochrome c: in vivo and in vitro effects.

Position 52 of iso-1-cytochrome c in the yeast Saccharomyces cerevisiae was systematically replaced with all possible amino acids to investigate the molecular basis of the "global suppressor" activity for the N52I mutation. Isogenic strains containing the variant proteins were made with a mixed oligonucleotide-directed mutagenesis technique in vivo. A relationship between thermodynamic protein stability and cellular protein levels was established by comparing direct spectroscopic measurements of cytochrome c levels in vivo with the thermodynamic parameters from guanidine hydrochloride denaturation and microcalorimetric measurements. Reversible denaturation data show that, within a large group of amino acid side chains, the thermodynamic stabilization at position 52 is related to the hydrophobicity of the side chain at that site.