Elimination of the disulphide bond alters the conformation of mature lipo-β-lactamase in yeast

When expressed in Saccharomyces cerevisiae the precursor of the hybrid prokaryotic protein lipo-β-lactamase is accumulated in reduced form whereas the majority of the mature form contains an intra molecular disulphide bond (oxidized form). We have previously shown that mature mutant lipo-β-lactamases in which the cysteine residue 131 was changed to either tyrosine or threonine lack the capacity to form a disulphide bond but are nevertheless processed and secreted efficiently in yeast. Here we show that these mutant mature lipo-β-lactamases, in yeast cell extracts, exhibit a tenfold lower β-lactamase-specific activity than the wild-type protein and that the mature mutant proteins are more susceptible to trypsin digestion. Thus, elimination of the disulphide bond alters the conformation of mature lipo-β-lactamase in yeast.