3.4.21.48 synthesis high-level production of human mechano-growth factor in Saccharomyces cerevisiae is interfered by proteinase B. Expression of mechano-growth factor in a proteinase B deletion mutant results in a fivefold increase of yield 718011 3.4.21.48 synthesis much stricter experimental protocols than those routinely used are necessary to prevent the artefactual interpretation of protein levels in strains or conditions that increase proteinase B activity. Stability of some proteins, such as Slt2p or Chs4p, but not others, is severely compromised in the rim101 mutant due to the upregulation of the PRB1 gene. Proteolytic degradation during protein processing can be almost completely prevented by an overdose of subtilisin-like protease inhibitors, such as PMSF, or by avoiding cell freezing 718456