Amino acid substitutions in PilD, a bifunctional enzyme of Pseudomonas aeruginosa -: Effect on leader peptidase and N-methyltransferase activities in vitro and in vivo

Subunits of type IV pill and a subset of proteins of the type II extracellular protein secretion apparatus undergo two consecutive post-translational modifications: leader peptide cleavage, followed by methylation of the newly created N-terminal amino acid. These two reactions are carried out by a single bifunctional enzyme encoded in Pseudomonas aeruginosa by the pilD gene. Properties of PilD mutants at positions Gly(95) and/or Lys(96) which were differentially affected in leader peptidase and N-methyltransferase function were characterized. None of the single amino acid substitutions showed a significant alteration in their ability to cleave the prepilin leader peptide; however, two double mutants did exhibit a modest reduction in the efficiency of cleavage. In contrast, a significant decrease of N-methyltransferase activity was detected in PilD having substitutions at Gly(95). Mutants with substitutions at position Lys(96) showed a variable effect on N-methyltransferase activity with an apparent requirement for any charged amino acid at this position. Absence of N-methyltransferase activity did not appear to interfere with the ability of P. aeruginosa to assemble functional pill. Moreover, pilin monomers isolated from P. aeruginosa expressing PilD with Gly(95) substitutions were not methylated. Although complete methylation does not appear to be absolutely required for pilus assembly in P. aeruginosa, this modification may be important for pilus function in its natural habitat.