Protease B from Saccharomyces cerevisiae. Purification and characterization

Protease B has been isolated from Saccharomyces cerevisiae and purified in six steps as follows: autolysis of the yeast cells, ammonium sulfate fractionation, activation of the proteolytic enzymes, chromatography on DEAE-cellulose, chromatography on CM-cellulose and finally, a second chromatography on DEAE-cellulose. The preparation was shown to be homogeneous on polyacrylamide gels in the absence as well as in the presence of sodium dodecylsulfate. Furthermore, the molecular weight (43,000 daltons) and the isoelectric point (5.45) were in good agreement with earlier published values. The amino acid composition is reported. The absence of disulfide bonds in protease B has to be outlined. The amino acid residues of the protein have been found to be folded nearly quantitatively (at least 80%) in a beta-conformation as deduced from a circular dichroism study. Finally, the tryptophan residues (5 mol/mol protein) are largely buried in the hydrophobic core of the enzyme.