Mutational analysis on stable expression and LasB inhibition of LasB propeptide in Pseudomonas aeruginosa

Three major proteases, elastase B (LasB), protease IV (PIV), and elastase A (LasA) expressed in Pseudomonas aeruginosa play important roles in infections and pathogeneses. These are activated by a proteolytic cascade initiated by the activation of LasB. In this study, we investigated whether LasB could be inhibited using its propeptide (LasBpp). Although LasA and PIV were inhibited by their propeptides, LasB was not inhibited by purified LasB_pp because LasB degraded LasB_pp. To address this problem, mutant LasBpp variants were constructed to obtain a mutant LasBpp resistant to LasB degradation. A C-terminal deletion series of LasB_pp was tested in vivo, and two positive candidates, T2 and T2-1, were selected. However, both caused growth retardation and were unstably expressed in vivo. Since deleting the C-terminal end of LasB_pp significantly affected its stable expression, substitution mutations were introduced at the two amino acids near the truncation site of T2-1. The resulting mutants, LasB_ppE172D, LasB_ppG173A, and LasB_ppE172DG173A, significantly diminished LasB activity when overexpressed in vivo and were stably expressed in MW1, a quorum sensing mutant that does not produce LasB. In vitro analysis showed that purified LasB_ppE172DG173A inhibited LasB activity to a small extent. Summarizing, C-terminal modification of LasB_pp profoundly affected the stable expression of LasB_pp, and little enhanced the ability of LasB_pp to resist degradation by LasB.