Abstract
Proteins resistant to proteinase K are rare because of the potency, wide pH optimum, and low peptide bond specificity of this enzyme. Previously, only the prion proteins associated with transmissible spongiform encephalopathies, possibly related proteins in the mollicute Spiroplasma mirum, and proteinase K itself have been reported. We identified a new proteinase K-resistant protein, p40-pr, in two strains of Mycoplasma hyorhinis and in extracts of these organisms. p40-pr's are similar to prion proteins in their resistance to high doses of proteinase K and in the reversal of this resistance by strong denaturing conditions. However, p40-pr's were distinct immunologically, in relative molecular mass, and in their method of extraction. Two immunologically related forms of p40-pr were identified on sodium dodecyl sulfate (SDS) gels and Western immunoblots, a 40-kDa species in boiled samples and a 120-kDa species dissociable by boiling in SDS. Reduction with 2-mercaptoethanol did not affect the mass of p40-pr's or the 120-kDa forms. The development of proteinase K resistance of p40-pr correlated to age-dependent increases in organism protein-lipid ratios. p40-pr-like proteinase K-resistant proteins of 46 to 50 kDa were identified in four of eight additional species of the class Mollicutes but not in S. mirum. However, these mycoplasmal proteins did not react with antibody to the denatured 40-kDa form of M. hyorhinis p40-pr purified by electroelution. The chromatographically purified 46-kDa proteinase K-resistant protein of Mycoplasma orale was an arginine deiminase.